@TPLDem Since you're on latest firmware, you could change the port config to 1000M instead of leaving it on auto as an attempt to have it stick. You might want to look up specs for the coupler or try...

@TPLDem If it automatically changes from 1000M to 100M, it may be a faulty cable that had to handshake lower due to errors/noise. I would suggest trying different cables for that port to confirm. The...

@DickLOliphant Some ISPs may limit how many IP they will give you. Your first router's MAC address may already be holding the only IP(s) they will give you. Try unplugging your modem for a few...

I haven't tried it but if you can split your network into multiple vlans then you could tag each incoming connection differently and allocate each one to a portion of the network.

@ADA21 First check if you are on latest firmware as mentioned by other posters above. Old firmware is most common issue with forwarding. If you are on latest firmware, you may be in the same position...