I recently solved a major problem with my Librem 5 phone.
For the entire time that I had owned the phone, I was not able to receive MMS messages. Regular SMSs arrived just fine, but no matter how many times I went through the setup procedure detailed on my phone provider's website and various Linux and phone guides, I could not get MMS messages to arrive correctly.
Eventually, I gave up. Most messaging happens on Signal and other platforms these days anyway.
However, after a recent experience where an important message almost didn't reach me, I dug into the problem again, and eventually found the traditional several-year-old forum post from a person who had had the same issue and actually solved it.
As it turns out (and I still don't completely understand why), the Librem 5 can only handle having one active APN at a time. Unfortunately, my cellular provider requires different APN settings for mobile data and receiving MMS messages — and while I had inputted both correctly, it had not occurred to me that I might need to tell the phone which one to use at a given time, or that the APN settings configured in the texting app might not actually be used.
I had never activated the latter, even though I had (thought I had) put the right settings in. Unfortunately, simply having put the right settings in was not enough — putting in the settings created the necessary APN profile, but did not activate it in a way the phone could use. After much searching and fiddling, I eventually discovered that the currently active APN profile had to be manually selected — and no other one would be used, even if correctly configured. Upon switching to the correct profile, several MMS messages arrived. Unfortunately, earlier ones could not be delivered (they expire after a while for understandable reasons), but I have a solution for the future at least.
Right now, I am leaving my phone set to the MMS profile (due to the expiration time), and only switching to regular internet when I want to check something specific. This is not ideal, but it gets the job done. Hopefully this will get fixed upstream at some point, at which point I won't need to do this anymore.
At least one person has come up with a programmatic workaround in the form of a script that periodically switches to MMS and checks for messages, which is hacky but perhaps better than having to do it by hand. I have not tried it yet, but there is no reason to assume that it wouldn't work; I will probably test it or something like it out when I have a spare few hours to mess around with my phone again.
All in all, this took several hours. While I could have probably found something more productive to do with the time, the end result was fruitful: I can now receive MMS messages to my Librem 5.