Connect Two WIFI Networks

At each site you will have a local area network (LAN) with a private IP address range, which you invent yourself.

I suggest you investigate a VPN tunnel linking the two LANs.

The private IP address ranges you set up in each LAN must be different, for example 192.168.1.0-25 and 192.168.2.0-25

Read the manuals for your WIFI/DSL routers and see what facility they have for setting up a VPN. It will help if the gateway public IP addresses of your two sites are static. If they are dynamic then you will have to change the configuration every time the public addresses change. This may occur if you power cycle your modems, or more often. You are looking for a cheap solution. If you can pay extra for fixed public IP addresses it would make life easier.

Use an encrypted VPN tunnel if possible and use complex long passwords for all equipments and the VPN encryption key. Change the passwords from time to time and whenever anyone who needed to know moves away.

Ref: https://en.wikipedia.org/wiki/Virtual_private_network

Once you have it all set up, a PC wanting to send packets to the internet will send them as before (straight 'up' to the public internet), but packets destined for the LAN IP addresses at the other site will go via the VPN tunnel.

Best regards, Eric.