It looks like there’s a max number of vlans that can be configured for the internal switch the TP-Link WDR841NDs have.
Related, but OpenWRT’s web interface can lie sometimes. The actual config file in /etc/config/network should not have an VLAN config IDs over 15, or it will fail to load them.
Confirm what has been loaded with
swconfig dev eth1 show and see what VLANs are actually being supported and loaded, where eth1 is the switch interface.
Also looks like there might be an issue with mixing tagged and untagged traffic on the same port.
This isn’t documented ANYWHERE on the ‘net, so I figured it was worth saving others some time.