Looks like you’ve got the Melco Audio original S100 - which has a history of physical layer connectivity issues!
In any case, that’s possibly what you’re also experiencing - a physical layer compatibility issue between your TP-Link and the S100 Auto-negotiation ports (5-8).
Just make sure your cable is not faulty or anything, and have you tried ports 1-3? Which are fixed 100MB. Your AP throughput capacity will throttle though. Not an issue for Mosaic, but whatever else you’re doing over WiFi might suffer.
Honestly, the Melco S100 is a lousy Ethernet Switch, you should dump it and get a proper Ethernet Switch (not any of the B.S “Audiophile” Ethernet Switches).