It sounds like you’re suffering from the problematic modem firmware update, of which there are several long posts in the last month.
Teltonika are working on a new firmware update but if you look at the other posts, there is a workaround and one of the posts describes the method to revert back to the previous firmware version. The downside is that to effect the ‘fix’, you will have to take the device home (or somewhere), where you can plug the Teltonika’s WAN port into a lan port of your home router with an internet connection.
But before taking the ‘plunge’, I’d wait until Teltonika support respond to your problem, as they may have other things to explore first.