No service on Blackview BV 5500 Pro with Lineage 17.1 The issue
I installed Lineage OS on my blackview BV 5500 Pro and am facing the &quot;No Service&quot; issue since. There does not seem to be any hardware issue and I've exhausted all the options I could think of.

Specifications

Image installed: lineage-17.1-20201012-UNOFFICIAL-treble_a64_bvS
Build number: treble_a64_bvS-userdebug 10 QQ3A.200805.001 eng.crossg.20201012.051017 test-keys
Carrier: Ultra mobile
Phone: Blackview BV 5500 Pro


What I've tried
Here's what I've tried:

Restarted the phone.
Activated the Airplane mode, waited, and de-activated it.
Set the values for data and MMS manually, using the carrier's recommendations.
Tried to select manually the carrier (more on that below).
Checked the sim card in a different phone -- it works.
Enabled the cellular data.
Allowed roaming.
Checked the phone with a different OS (it was working just fine before I flashed the OS).
Made sure my sim card was correctly inserted.
Called *#06# and checked that my sim had an IMEI / ESN (it does, I actually have two since this phone accepts two sims).
Tried the other sim slot.
Called *#*#4636#*#*  and checked the information about my phone:

It does have a IMEI and IMSI,
I can ran a ping test,
Radio band is set to automatic,
Selected GSM / WCDM, LTE only, and other various network types,


Re-installed Lineage.
Re-installed Lineage with GApps nano.


Possible issues?
Manually select the carrier
When I try to manually select a carrier, I'm faced with cryptic choices (&quot;310830&quot;, &quot;311490&quot;, &quot;311480&quot;, …), and all of them returns an &quot;impossible to connect to this network&quot; error message.
Comparison
I have the same phone with the same carrier at home, but with the stock rom on it (Android 9, build BV5500Pro_E535_V1.0_20191105V6). If I check in *#*#4636#*#*, I can see that the Network Mode is set to LTE / UMTS auto (PRL). However, that choice is not available on my phone.
Baseband?
Between the stock rom  and Lineage OS, I've tried another rom (BV5500Pro_EEA_E535_V1.0_20190514V8) to test that I could flash my phone. Could that have messed with the baseband, or something else? My current baseband is E535_39_P0_LWTG_V0.1.4_S190402.
Update
So, actually, I've tried to install magisk, which I believe is the culprit.
Following the documentation, since this model does not have Ramdisk memory, I've patched the recovery.img file using their Manager application, and then flashed my recovery partition, using
 fastboot flash recovery /path/to/magisk_patched.img

After rebooting, their application was still not detecting magisk's installation, and my OS could not detect my SIM card anymore.
After flashing back the stock recovery, the sim card was back.
This seems to be a fairly common issue, judging by

https://github.com/topjohnwu/Magisk/issues/2201
https://forum.xda-developers.com/mate-9/help/sim-card-reading-failed-magisk-xposed-t3819966
https://forum.xda-developers.com/apps/magisk/problem-sim-card-detected-constant-t3860135


Original Message
Ok, so I've done the following:

Installed the official rom, BV5500Pro_E535_V1.0_20191105V6, that blackview shared directly with me: the sim card was not even recognized!
Flashed everything, installed a previous version of lineage (lineage-16.0-20191017-UNOFFICIAL-treble_a64_bvN): sim card still not recognized.
Installed another official rom (that they share here, called BV5500Pro_EEA_E535_V1.0_20190514V8): the sim card is back, and I can connect to the network!
Re-installed lineage-17.1-20201012-UNOFFICIAL-treble_a64_bvS. The sim card is still detected, and I can now connect to the service!

Needless to say, I have no clue what actually solved my issue.
My network is set to GSM / WCLMA / LTE (PRL) and I have not noticed any value or configuration that would have changed…