Vivaldi browser not syncing over LAN but can sync over WIFI

[ByteBiter]

Hardware: RAX200

Firmware: V1.0.10.140_1.0.79

I use the Vivaldi browser, which syncs its settings across computers. I have been running into the following issue.

I have multiple computers connected to the same Nighthawk router. Syncing fails on the one computer connected using LAN. The computers that use WIFI have no problem.

I can confirm that the router is the problem, as plugging the problematic computer into the network directly allows my browser to sync its settings.

Is there a router setting (specific to LAN) that might prevent some processes from reaching the web while the same services would run OK when connected to WIFI (on the same router)?

[FURRYe38]

Try disabling the following and see:
Armor, Smart Parental Controls or Circle, Traffic Meter.

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

Has a factory reset and setup from scratch been performed?

[plemans]

might check on that pc if you have any security, vpn, or ethernet settings. 

Have you tried the other devices wired? 

Or have that pc set with a public address and not private when on ethernet? (check in your windows settings)