easy. firmware is posted on the official website: https://www.tp-link.com/en/download/TL-WA850RE.html#Firmware The firmware must match its hardware, otherwise it may ruin this device. So keep it in...

right, the capacity of the wired device should be taken into consideration.

model number? any pictures showing the energy usage? more details will be better.

what is the hardware of your device? V1 or V2？usually the firmware should match its hardware, you'd better to confirm it first.

wireless router mode and 3/4G router mode are separately, cannot exist at the same time. So may not able use 4G internet as a backup when it works as a wireless router or access point.

so cannot detect the TP-link wifi on your printer, right. Go change the wireless settings of the router and give it a go, epsecialy to change its wireless channel, like channel 1, 6 and 11.