VPN for customer location without port forward etc


I install audio systems , IP cameras or PBX for customers, is there a solution for VPN where I can connect to the network remotely without having to put their modem into bridge mode and then VPN into my router?
I would like to avoid messing with ISP modem or port forward. (If ISP changes modem or something so I don’t need to go to customer location to change settings.)
Anything with raspberry Pi ? Any router?
My customer is usually a restaurant so there is no PC for me to remote in.

Thanks for any input.

You might want to look at tail scale

+1 for Tailscale. Another option is to use IPv6. No need for NAT when your secondary router has an internet-routable address itself. Granted, the primary router’s firewall needs to allow the traffic, so it might require configuration.

If you want to do it with wireguard, just have the customer box (assuming it’s pfsense) be the “client” calling into your “server” on your end. End users (clients) can roam all they want and be behind all they want. Just need to be able to reach your server.