I did tests on Brume 2 as both WireGuard and OpenVPN server. The Brume 2 LAN and main router LAN will be on different subnets and have separate DHCP servers. However, the WireGuard client device (and any devices directly connected to the Brume 2 LAN) will be routed through to the main router LAN subnet by using IP addresses (not NetBIOS names). Internet access will also be routed through the main router to your ISP.
I do not work for and I do not have formal association with GL.iNet
I mean, it sounds like what you want to do is just forward a port through to the Brume 2, then route 0.0.0.0/0 to it from your mobile devices. You could do this with… well any Linux client/VM, really, but a Brume 2 is as good as anything else I suppose.
Basically set up a VPN server on the Brume 2, pass the appropriate port through on your main router such that traffic arrives on the Brume 2, then on your client VPN settings route all traffic. You’ll be able to access everything on your LAN (because all traffic will look like it’s exiting the Brume 2) and