If you have line of sight to nearby buildings, then perhaps you can use a narrow beam Ethernet radio links to do the inter-building link.

This stuff is often used to link nearby buildings in campus and business districts.

Each antenna is pointed at the other, and both have a very narrow field of view, so they only 'see' each other. Even if other gear in the area is using the exact same frequencies, it won't have much effect on your radio link.

Some of this gear isn't all that expensive, especially if it uses the unlicensed radio bands (some of the same frequencies that WiFi uses, plus some others like 900Mhz).

Essentially you plug an ethernet cable into each box, and the radio link acts link a long ethernet 'extension cord' between the two ends.

Connect one box to the Internet-connects-here port on your building's router, and plug the other radio box into the internet 'source' in the other building.