Hi Chester,
Sorry for the late response. I believe I have the same MMS bug as you. Oneplus 5T, still running 16.04 (daily driver, can't afford glitches). I've never been able to figure out what causes MMS to stop sending/receiving. I get no notification that I've been sent anything when it stops working, so it's hard to know that anything's wrong.
But once I know, to get it to work again, I go to terminal and type:
initctl --user restart nuntium
Works every time. For me I'm able to test whether it's working or not by sending a photo to myself. If only I could know when it stops working, I wouldn't occasionally miss messages entirely.

My experience is that if my MMS isn't working when somebody sends me one, it will arrive exactly one hour later if I've corrected it by then, or exactly twelve hours later, or exactly twenty four hours later. But if I didn't realize and haven't corrected the problem by 24 hours later I'll never see that message or know that it was sent.

Anyhow, people have worked hard to get MMS working as well as it does, and I'm truly grateful. Don't like to sound complainy.

If yours starts being glitchy again, I hope the restart nuntium can help.

I finally managed to get it working. Thank you all for your help and advices. 🙂

To do so, I used usb with MTP to transfer the ssh public key file generated on my computer to my OP5. After that I used the terminal on my phone to manipule files and folders using the phablet account. Note that this account seems not present in recovery mode.
I followed the chapter 'configure your device' on that page, turned the wifi on, and enabled ssh.
Now the ssh connection is working fine. I can now use the clickable --ssh option the develop on my OP5.

Again, thank you all!! 🙂