Viktor, how exactly did you do that, to "protect PPP link from garbage packets"?  Just an iptables OUTPUT rule?

I'm having the same issue with a UML295.  At first I chased the red herring of thinking this was the cause:

Oct 29 16:09:33 tp pppd[19330]: Protocol-Reject for 'Compression Control Protocol' (0x80fd) received

But I get the modem hangup even with "noccp" in pppd options.

My device works fine (yes, Alicia, 100% of my account usage has been under Linux, regardless of what Verizon is willing to say they "support") using cdc-ether and DHCP, but I am wishing to switch to an internal modem which does not have the cdc-ether option.

Also, Viktor, if you can share that chat script it would be nice (but I guess it's all there in the logs you showed, I will play around with that.)  Thank you.