Hello All and @notjuraj - many stuff are detected and fixed.
I just found that case and try analyze it
Fist posts say about SXT LTE 3-7 who is old hardware and his fw version is like “chip:2.0 fw:0.1.2.4”
Some other post refer to SXTR with R11e-LTE who is know as SXT LTE Kit with firmware like MikroTik_CP_2.160.000_v018 and this hardware is under support, we can check that problem.
This 3-7 or Kit in the product name is very important.
Currently the disappearing lte1 is know for me at some SIM card (Poland Orange) with only r11e-LTE6 who is in loop of reboots - this case I know and currently one unit will be share to support.
Problem what you describe is like solved for me, or any one not write about it recently.
If anyone have that problem please contact with me.
Good way for monitoring is a script Watch LTE parameters: http://forum.mikrotik.com/t/r11e-lte6-2ca-not-staying-connected/136004/22 but he track working lte1, I never have a case with problems with lte1 interface itself.
About detect no internet and do some action I use that ways:
Simple watchdog: http://forum.mikrotik.com/t/sxt-lte-4g-cat6/134468/1
Advance watchdog: http://forum.mikrotik.com/t/tx-rx-fp-rx-dropped-pppoe-account/132656/1
BR Marcin (SiB) Przysowa