I have a Synology RT2600ac (main modem/router) and a Synology MR2200ac (mesh node) which I am trying to connect together via ethernet.

ONT > ethernet > RT2600ac > ethernet > MR2200ac

The RT2600ac works fine by itself, devices are connected via LAN and wifi just fine. The MR2200ac even works fine when it's close by and uses the 5GHz uplink, but it won't connect via ethernet at all. If I choose ethernet-only uplink it stops working so usually I leave it on auto and it falls back to the 5GHz uplink. The 5GHz connection isn't sustainable long term as the devices are usually a long way apart.

Both routers are new and so is the ethernet cabling so I don't think those are the issue. The Synology routers ask you to set up VLAN tagging per-port and I've tried turning it on and off for the port which the MR2200ac is connected on but it makes no difference. Here are the tagging settings:

Interestingly I have found a (less than ideal) workaround: if I put a third party modem before the RT2600ac (enabling VLAN tagging on that modem and disabling tagging on the RT2600ac) everything works great! The MR2200ac automatically connects via the ethernet uplink which is what I'm after.

ONT > ethernet > Third-party-router > ethernet > RT2600ac > ethernet > MR2200ac

The third-party modem is old and slow, and I'd rather not have to maintain thee routers in the house! I'd really like to get the RT2600ac and MR2200ac working together correctly without a third-party device. But perhaps the above workaround can help point us towards the answer: the VLAN tagging perhaps? I've tried toggling tagging on and off for then port which the MR2200ac is plugged into (LAN3 or LAN4) but it doesn't help.

Has anyone else had success with this setup? Can you offer any advice for troubleshooting the issue?

I'm using the latest Synology firmware: SRM 1.2.4-8081