Hi there,

so a couple extra things you could try is 1. Resetting your network settings. 2. Taking out the sim and re-inserting it 3. Replacing the sim all together and see if it works and 4. Setting LTE as your preferred network in your settings. See if these work and let me know

Before switching the sim, I would try the sim in another working phone and another working sim into your phone to to see if the issue follows the sim or the phone.  You can do this with a friend or family member

I got fed up with the nonstop notifications so I ended up requesting a callback. Spent a good deal of time trying to troubleshoot - involving resetting the phone/network/etc. Nothing worked. Phone still kept flipping between HSPA and LTE. VOLTE refuses to work.

I think the problem might be related to the latest LineageOS21 update (april 29). I’ve read that LOS was supposed to be making modem changes to accommodate Version (simialr HW to Telus IIRC) issues - yes, I know, my fault for using 3rd party firmware

If I remember back to LOS20, there were data issues if people were using a physical SIM, but supposedly were OK with eSIM. Anybody have any experience on that front? Wondering if maybe a switch to eSIM would help

You will have better luck asking questions  about LineageOS on the XDA forums.