Well, I finanally found the firmware here: https://www.tp-link.com/us/support/download/tl-mr3020/?utm_medium=select-local#Firmware I reinstalled the current firmware, down to the first. Same issue...

My TP-Link ML-MR3020 is constantly rebooting anytime I insert the 4G LTE USB Modem 551L. I have verified that the modem works on computers. This web pages says the two are compatible:...