It depends somewhat on your existing network. The Brume 2 has only Ethernet LAN/WAN ports and does not have wifi. If you do NOT require wifi, then it is straightforward to add the Brume 2 behind your main router as a 2nd router layer, or you can bridge your main router so the Brume 2 acts as your main router.
If you require wifi that your main router currently provides, then the Brume 2 WAN port can be connected to a main router LAN port, then set up as a VPN server, but not with AdGuardHome at the same time:
For AdGuardHome, but not VPN server, the Brume 2 LAN port can be connected to a main router LAN port:
I have not personally used the Drop-In Gateway capability, but it may be able to do either and/or both functions. Maybe other forum members have experiences they can share.
Here is the GL.iNet documentation on the Brume 2 VPN and AdGuardHome functions: