More than you want but I don’t want to repeat it all:
VPN instructions for a newbie | SmallNetBuilder Forums

Accessing the LAN side of the Asus router is a function of pushing a route to the remote device so it knows how to reach the LAN side. Whether you reach the internet through the tunnel into the Asus router, and then out from there, or alternatively, not through the tunnel at all, is a function of whether the server is pushing a default gateway over the tunnel, and whether the client is accepting it or ignoring it.

Also: if any of the nets along the way have the same subnet, you will be messed up. Asus has moved from 192.168.1.x to 192.168.50.x, for example, and GL-inet is using 192.168.8.x, and you want to avoid 0.x or 1.x since they are likely to be in your mix somewhere. This can come into play unexpectedly in your path, for example with a DSL or cable modem.