on iPhone, install the Apple Airport Utility if you don't have it. Then, in Settings > Airport Utility, enable WiFi Scanner.
Now, open the app, don't worry that there are no access points. In the upper right corner, click Scan and let it run for a minute, then stop it.
What you will see is all of the WiFi signals in range of your current location. It will show channels these use, and a signal strength (RSSI). RSSI readings closer to 0 (-30 for instance) are stronger than -90. Anything over -80 you can ignore, these are too weak\far away to interfere with your network. Jot down all of the network signals below -80, these are the most likely conflicts. You will see your own in there, probably something for 5Ghz like channels 36 and above. You will also see 2.4Ghz with channels 1-11 or so. As you write them down, do so on a graph=like line with ch 1-11 for 2.4, and ch 36 -159 or so for 5Ghz. Then, along that line, tick where each signal is on the spectrum to easily find gaps where you will set your channels.
5Ghz has shorter range, you may not even see neighbors networks with channels 36+, but if you do, note them. 2.4Ghz is generally more problematic, there are only a handful of channels, and range is longer so you will likely see neighboring networks, again note the channels with these.
When tuning 2.4Ghz channels, you generally want to avoid channel 6, it is where all the garbage signals occur. Microwave ovens, radar, bluetooth, wireless home phones, wireless keyboard\mouse will generally be using this frequency\channel. If you choose channel 1, you will actually use 1-3. If you choose 3, you will use 2-4 or 5 and so on. Basically, 3-4 channels are used when you set a channel on the router or extender. So, you are looking for where in the spectrum the stronger neighbor signals are, and trying to fill gaps with yours to minimize overlap. Same with 5Ghz, but that may not be as saturated and there are more channels to play with.
It's a game of pin the tail on the donkey, if neighbors know how to do this, channels will be changed from time to time, and stomp on each other. To understand what you are trying to do here, think of AM and FM radio. AM is like 2.4Ghz, it has longer range due to longer signal waves (lower frequency). As you move between cities, you may pickup two broadcasts from radio stations using the same frequency (channel), so you hear two sets of voices or music, and have to try to filter that with your ears. But, if the neighboring city has no AM stations near the frequencies of nearby cities, there is little chance of overlap, just static from weak signals (you are not getting all of the data or hearing electrical interference). More often, at night (limited light to interfere with signals), the overlap occurs because longer distances for the signal to travel before it is too week to pick up.
So, changing WiFi channels is like telling 1 radio station to use 89.3 instead of 89.7 to avoid overlapping. FM (and 5Ghz) is higher frequency so it doesn't travel as far, but same idea. So, you are tuning your network to broadcast and listen on certain frequencies. Clients learn which channels to use from the source when they join the network.
As I think I mentioned, WiFi 6 (802.11ax) has a filtering mechanism in which it sees a bit in the header of each packet that indicates which network authenticated it. So, the router or extender can more easily drop these without deeper inspection to make room (priority) for authentic packets. Thus, it is less prone to interference from neighbor networks. But it is new, more expensive, and few devices use AX yet (the next 1-3 years, most new devices will support AX).
Before you spend money on AC WiFi gear (WiFi 5), try your best to live with it and tune it for now. When WiFi 6 prices drop, and devices support it, then may be a good time to look into investing in new equipment, that may be in 2-3 years. WiFi 6 also has longer range, faster speeds, in addition to resilient to interference. So, a single WiFi 6 router may cover your whole home without an extender.
By now, you are probably thinking...jeez, how can this ever work. Thus is the downfall of WiFi. I generally try to hardwire as much as possible. This reduces the number of WiFi signals bouncing around. Think of a highway, today with everyone sheltered in their homes, few cars on the road, so going the speed limit is easy. But in normal rush hour with too many cars, things slow down. Taking cars off the road speeds things up for everyone, in your lane (channel), and surrounding lanes.
I think you have two Ethernet ports in the extender, probably 4 on the router. If you connect a 4 or 8 port switch to one of the ports, you can connect 4 or 8 devices to a single port on the router\extender. Generally, you can use up to 3 or 4 switches on a single run of cable before things get mucked up. So, Router > Switch > Switch > Switch > PC\Mac\Streaming device, etc. Each switch can have several ports connected to devices...this is how you build out larger networks like in offices. In your case, MOCA is the medium (wire) that carries the data, and the Extender is effectively a switch. So, one switch connected to the extender to get ethernet to several devices would work well, particularly if the MOCA connection is good.
Back to your MOCA. Your router is MOCA 2.0, bonded I believe. Think of this like a 1Gbps ethernet connection. The extender is MOCA 1.1, I believe. This is basically a 100Mbps connection. If you only have a couple devices connecting through the extender, 100Mbps is probably more than enough, even 4k video streams are not more than 15-20 Mbps. But, the filter might be lowering the extender to router MOCA connection to 50 Mbps or so due to filtering out higher frequencies. The new splitter should bring the MOCA link up to 100Mbps or better, thus improving things considerably.
Once things are working reasonably well, consider using different WiFi names on the router and extender. Like NETWORK and NETWORK_E. For devices that are near the extender, only connect them to the extender WiFi, devices near the router, to the router WiFi. You can even do NETWORK\NETWORK5 and NETWORK_E\NETWORK_E5 to split 2.4 and 5Ghz into different networks. This allows you to join the best network in a given location for a given device and not roam to the others and cause congestion for other devices. For things like iPad, iPhone that roam, join all of the networks and let the device sort out the best to use.
Is your head exploding yet?
