There are two ideas:
You say they do not touch. However, if they have line-of-site, you could look at wireless point-to-point Ubiquiti M5’s (we use model NSM5). Here’s a link: https://www.ubnt.com/airmax/nanostationm/ I have this setup at one of our campuses where two buildings are not touching, but needed internet connection to the second. The second building was too small to put in the cost, time, and effort of running fiber… but for about $100 per side, we set this up and it works great!
The second thing is, if they don’t have line-of-site, you could do a site-to-site vpn via the firewall. We have six locations connected together using site-to-site vpn’s. As mentioned above, you’ll need your firewalls to have a static IP from the ISP, and then you can set it up. We use Cisco ASA 5505’s at our Regional Campuses, Cisco ASA 5510 (upgrading to a Cisco ASA 5525-X this year) at our main campus. There is a VPN Wizard from the ASDM (graphical interface), to assist in site-to-site vpn creation, with traffic flowing between campuses. I know others on the form use other firewalls and can speak to those.
Hope this helps!