After some 5 years of service the LTE modem stopped being detected by the OS.
The SXT LTE was mounted in the focal point of a parabolic dish that looks at the horizon, so only slightly tilted backwards vs. normal mounting of a SXT housing on a pole.
Have now removed the unit from its remote location and opened it. Surprise: after removal of the rectangular sheet metal cover of the R11e-LTE modem, the side opposite the connectors, there was water covering the biggest IC in the modem, that happens to be at the lowest point inside the modem.
Clearly the SXT housing, with its long and unsealed connection between the bottom plastic cover and plastic covering the antennas, is not sufficiently water tight and lets humid air enter the housing. Or at least after some years that cause the plastic to become more brittle.
Would recommend, before mounting a SXT, to cover the rim that connects the two encasing halves with some UV resistant tape, only leaving the little bottom ventilation slot open.
Once water has accumulated at the lowest point of the R11e modem due to condensation, it cannot go anywhere as the tiny ventilation openings in the sheet metal cover sit tight against the black rubbery heat conduction slab that sits between the modem and the SXT PCB. So at least two recommendations for mechanical improvement: 1. seal the connection between the two housing halves 2. allow for some ventilation and water release from the R11e modem side that sits against the heat conducting slab.

update:
as the Routerboard inside the SXTLTE still operated normally, just not detecting the LTE modem in the mini PCIe slot, I took a gamble and just ordered a new LTE modem, LTE6 this time, i.s.o. a completely new SXT LTE. The LTE modem that accumulated moisture did not revive after drying.
Upgraded the SXT mother board to RouterOS v7.11 and included the LTE package. The new LTE6 modem is recognized by the old SXT mother board (screen silk production year/month 18 11) and works fine. Have removed some of the rubbery black substance right next to the PCIe connector to allow for a way out for any future moisture.