iMessage only working with Wifi (not cellular)

[macstatic]

I'm having trouble getting iMessage to work with a cellular connection (it works fine when Wifi is turned on however).
If I try to turn it on (Settings-Messages) without having Wifi turned on I get the following error message:

iMessage unavailable
Turn on mobile data or use Wi-Fi to set up iMessage

[Settings] [OK]

If I have wifi turned on and try to turn it on the same way I'm allowed to do this without any error message.
I suspect it may have something with not allowing iMessage to access the cellular connection (which makes sense since I've tried to limit most apps to do this as I've had a prepaid plan for my phone until a couple of days ago). I have Mobile data turned on, but among the apps listed there none are enabled.
I expected to see "Messages" among the apps there, so I could turn it on, but it's not there! Is there something wrong with my setup?

I'm on iOS 15.7 using an iPhone SE (1st generation).

 

[macstatic]

I think I've figured it out myself....
It appears that you can only turn on/enable iMessage on the iphone with wifi, which pops up a message on my Mac asking to confirm if I want to add my iphone to my iMessage account. Once that's done I can turn off wifi and continue using iMessage on the phone with just cellular (until I turn iMessage off and then on again).
I wish Apple had explained this somehow....

On a sidenote, what does enabling "Phone" (Settings-Mobile data) among all the apps listed there do? I tried this as well, but obviously it had nothing to do with allowing iMessage to use cellular data or not.

 

[0128672]

It almost sounds like you're making this more complicated than it needs to be. I would simply leave both cellular data and mobile data on. As I mentioned in your other thread on a related topic, your wi-fi network will be prioritized over your cellular/mobile.

Set up Messages on iPhone

Set up Messages and iMessage on iPhone to exchange text messages and other media.

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View or change cellular data settings on iPhone

On iPhone, turn cellular data and roaming on or off, set which apps and services use cellular data, and set other cellular options.

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Use cellular data on your iPhone or iPad - Apple Support

Find out how much cellular data that you're using.

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