The 4th port on your router is the only one that can work as a way to connect to the ONT , it can be ‘switched’ between being a LAN or WAN port by entering the router admin pages and turning FTTP mode on ( you should have already done this to turn port 4 from LAN to WAN ) , as already stated the colour of the plugs on the Ethernet cable don’t matter , using a different cable is just to eliminate the cable as a possible fault
….if the router is on , it’s menu option has been switched to FTTP mode , a different cable used between the ONT and router and the ONT LAN light still doesn’t come on , and BT have confirmed your account is correct configured then it’s most likely that the router is at fault ( even though it works when used on FTTC ) or the ONT is faulty.
If your computer is a desktop it will have ‘Ethernet’ , ( WiFi would be useless in this situation anyway ) , if it doesn’t have WiFi presumably it’s pretty close to where the router is placed , (otherwise how did you use it when you were on FTTC ) if the ONT was positioned nearby , then connecting the ONT directly to the PC may not be too difficult , try to connect its Ethernet port to the ONT and set up a PPPoE session , if you get online , it’s definitely not the ONT but the router , if you still don’t get online the ONT may be at fault.
If it’s impossible for you to do the PPPoE , then have you been offered a new router ?, were you offered a new router when you upgraded to FTTP ?,
If you have done factory resets on the router have you checked the FTTP ( full fibre mode ) is still ‘ON’ ?
