As at the time if the original reply it wasn’t known what your ‘setup’ entailed, the only Ethernet cable that was known to exist was between the ONT and router, that’s the first one to try swapping out to eliminate it as the problem, you could also look up how to set up a PPPoE session on your PC, linking direct from from your ONT to a PC/laptop (assuming you have a one with Ethernet ) some modern ‘slim’ cases don’t have an Ethernet socket , but you can get adapters, using a different Ethernet cable to the existing one between the ONT and router , then you can also eliminate the router as the potential problem .
