Without more information like VPN provider, VPN setup on client or router, etc. one can only guess. Check the VPN setup to see if there is an option to either include the local network IP subnet in the VPN settings or a toggle/switch to allow for local network access while the VPN tunnel is active.
If you haven’t done so already, do an internet search of your VPN provider and Linux. There is bound to be directions on how to adjust the VPN client settings to allow for local network access.