Slower WiFi is common, there can be many causes.
That model supports 802.11AC (WiFi 5) 5Ghz and 2.4Ghz. By default, both bands will use the same credentials (SSID and Password). Wireless devices will choose a band based on best signal, but may hang on to that connection even if you roam to a weaker signal location.
5Ghz covers a smaller areas, it is subject to more signal loss due to both distance, and passing through solid surfaces than 2.4Ghz. 2.4 is generally slower than 5Ghz. 2.4 also suffers from interference from a lot of sources, including microwaves, radar, bluetooth, wireless home phones, etc. Because the range of 2.4 is greater, you will often have overlapping networks in congested areas, your neighbors routers will be in range and if using the same channels, both networks will see traffic and have to sort which is authorized. This can cause dropped packets which need to be re-transmitted...this is what slows things down.
One thing you can try is to name the bands something different, say NETWORK and NETWORK5G you can use the same password). On the iPhone, "forget the 2.4Ghz network, and join the 5Ghz network. Now test speed in the same room as the router. Repeat in other rooms and you will get a sense of what range will work for the iPhone. As you move farther from the router, speeds will drop, that is normal.
Then, forget the 5G and connect to the 2.4G network and re-run the tests. You should see better speeds at distance unless there is a lot of interference. But, speeds may generally be less than when connected to 5G until you get further away.
The baby monitor likely uses 2.4Ghz, so yes that could cause some interference for the 2.4Ghz band.
Last, you can manually set channels on the router. If you install Airport Utility on your phone, and enable WiFi scanner in Settings > Airport Utility, then click the scan link at the top right on the iPhone AP Utility, you will see all the networks and channels in range of the phone. Choose a block of 3 channels with the least overlap with neighbors to use on the router settings. Repeat for 5Ghz. The objective is to tune your network to different frequencies (channels) than your neighbors to reduce collisions. Think of AM radio, as you move away from the city, the signal gets weaker, and you often pickup the signal from another city on the same frequency, so you hear both broadcasts. That is similar to what WiFi is doing, but the router has to inspect and then toss packets if they are from unauthenticated devices.
If the neighbors are flooding all of the channels, look for the block of channels with the weakest signals and use that on your router. the channel you choose will actually be +- 1-2 frequencies, it is really a range of channels. Channel 6 on 2.4 is where everything defaults, so often using 1 or 11 will help.
It's a pain, but that is the nature of WiFi. Good news is, the latest WiFi 6 (802.11AX) is designed to filter out neighboring signals to effectively give your devices more priority to access the radios on your router. Apple doesn't have a WiFi 6 router, and it is still relatively new so a bit pricey. Cost should come down this year or next as people upgrade and more devices support WiFi6.
