Can't connect to router from a client on VPN

I've setup a L2TP VPN on my Synology router (a RT6600ax). The VPN works great and when connected to it, I can access everything on my local network... except for the router itself. I can't ping the router's internal IP, I can't open the router's web UI, I can't even access the router through the DS Router app on my iPhone (when that's connected to the VPN).

The IP address my host receives is correct (192.168.0.xxx, my router's internal LAN IP is 192.168.0.1). At the same time, if I connect to another machine on my local network (via say RDP, SSH, whatever), I can access the router's WebUI through that with no problems.

I've been pouring over all of the settings on the router, in the Network Cnter, the main Control Panel, the VPN Plus Server and I can't find anything which could help.

Does anyone have any ideas before I go insane? The router is running the latest SRM 1.3.1-9346 Update 2.