That is a sort of an unsurprising problem (which I understand) with an unexpected response.
I have been tinkering with the fish shell and one of my co-workers often uses zsh. However, I have simply installed them and haven't even changed the default away from bash. We just call them manually when we want to use them. Thinking about when things work and when they don't, I am unable to confirm if I have been running fish when they fail or if I am still in bash when they work. (I'm assuming their mere presence in the system isn't a big deal). I will have to test this theory to have a more reliable understanding.
I have the impression that Modem Manager is intended to be used, with a GUI, in combination with Network Manager. Also, I read somewhere that Network Manager was designed for use cases like laptops and wasn't really intended to be used in headless servers... although thinking about it, I guess there aren't many people trying to set up servers in locations out in the field with terrible internet service... so perhaps that advice doesn't really cover my use case.
Searching around, I have been unable to find a usable example of using Modem Manager from the command line... so I am beginning to suspect that "using Modem Manager" winds up meaning "using Network Manager in combination Modem Manager", is that the case? This evening I will try to install Network Manager & Modem Manager and do all of this in the "Network Manager"... but I am not sure if that is an appropriate long term solution (or even why it wouldn't be).