I did contact Synology but it's an older model out of warranty and apparently they don't have a repair shop.
I did try the troubleshooting steps you mentioned. The problem is, it's not like a normal NIC card that you can replace, it is built into the unit and no easy way to swap out as you most likely already know.
The only other option I think would be to plug in a wifi dongle to use as a backup means of communication. Not the best solution but the only way I can think of to have backup comms. I know this isn't supported anymore but I'm out of options.
Any recommendations for a wifi dongle?