how about release and renew your IP address and have a try? you can notice whether IP address for your PC is same segment for TL-R480T+ in advance.

the port is red for WAN? I checked the UG and only Green or Yellow is available for the ethernet port. When disconnect, how about the status showed on interface, maybe you can also have a try for MAC...

Seems this problem existed in SF1008P V1, I suspect the Power management is static and defined by PoE class so that it could beyond the max power consumer easily.

TP-LINK support had told me that it can work with TL-SG5428, so I think it could work for SG3424 as well.

I think you can use another PC to test and also notice the ping latency.

If only one client to connect, you can set the IP pool as only one so that this is also "fixed" IP, otherwise I don't think it can meet your demand.