I must be drain-bamaged

I am having a bad day and think my recent brain injury is causing me to fail to understand the proper setup of my Brume 2. Could you help me understand what I need to do?

Currently working network set up:

Motorola MB8611
TP-Link Deco M4

I will be adding the Brume 2 to this system. Do I place it between the modem and the Decos or after the Decos? Do I setup port forwarding on the modem or on the Decos if I have the VPN connecting to the Deco and not directly to the modem?

I want to only to allow a client to connect to the network from abroad and use the LAN’s ISP connection.

I am really struggling mentally today.

To keep it simple, you put the Brume2 in between your modem and Deco, allow the modem to be just that (i.e. a modem), let the Brume2 be the main router (doing all the fancy things including VPN) and use your Deco in Access Point mode only (i.e. just a dumb WiFi access point). Finally, I hope you stay well.