Did you find resolution?
I have hAP ac2 also and have similar issue. After upgrade to RouterOS 7 my USB modem (huaweii 3372) stop work. Modem was recognized by router I could found it in resources, in ppp interface I could check info details but do not establish connection.
On my device it always was seen as ppp client interface (not LTE), but do not work. In status was "waiting for packages".
After I downgrade to latest 6 version all works as earlier with out any issues.
Any idea what could be wrong?