I still have no information on FG621-EA modem in HAP ax lite lte6, but it listed as supported for external esims here
https://help.mikrotik.com/docs/spaces/ROS/pages/30146563/LTE#LTE-ModemeSIMmanagementsupporttable
But t77w968-esim and t77w968 with 9esim modems with mikrotik - fully functional!
I can add/delete/switch any esim profiles with winbox.