When comparing Turris OS 6.5.2 with 7.0,0, I found that the ModemManager package in 7.0 actually uses an older ModemManager version (7.0.0 has version 1.18.6 whereas 6.5.2 had 1.18.12). I filed a bug report for this:
It’s hard to tell if that really is related to the issues you’re experiencing. But it might be worth to try the newer version which is already part of the 7.0.1 RC You can find it here. I make no guaranties, but since the differences between 7.0.0 and 7.0.1 are minor, it should be possible to install the package from the downloaded ipk file (even without switching to the testing branch and upgrading to TO 7.0.1).