I have setup OVPN on my mirkotk router and setup OPENVPN on my PC. I can connect to the OVPN server from the PC on the OVPN network of 192.168.20.0.24 and no issue. But, I can ping or access any equipment on the LAN network on 192.168.3.0/24.
Create IP firewall filter rule for forward chain with src. address 192.168.20.0/24 and dst. address 192.168.3.0/24 (or set in interface list to LAN) and action accept.
You can add additional reversed dst/src rule if you want access from LAN to VPN.