I do have a quite similar difficulty with SCS card modem "TRXPTC". My observation is, that the modem is waiting for a reply to several OPEN commands and then disconnecting. I checked the Log for the relevant port and found this:
Port(5.354):2020-02-07T11:57:50.354 Missed Expected Response(cmd:) From Command(QUIT!cmd:)
Port(5.354):2020-02-07T11:57:50.354 Terminating after 60496 msec vs 60000 Quiet
Maybe you check your log as well? Is it the same/similar message showing.
73,s Wolfgang OH6DL