Switch problem
First of all, you can try to switch your PBX with switches in other places to see if it is a switch problem.
Network cable problem
Besides, you said that the network cable can be used. Have you actually tested it? If the PBX has been replaced, but the problem persists, it may be the network cable or the main switch port.
Signal lamp problem
Why don't you try connecting it to the port of PBX in other places and see if the signal light of PBX is on? If it doesn't light up, it must be a problem with the small exchange; If it is open, there may be something wrong with your line.