Sharing internet between two buildings, what am I missing?

I am currently setup so that one server in the second building is able to have the internet shared via its second nic and a ubiquiti nanostation m5 to link the two buildings together. the m5 is directly connected to the second nic and on the other end, that m5, is directly connect to the modem. This setup is working fine.

What i'd like to do now is expand the second building so that all PC's can have the internet shared. Both buildings are on separate subnets and they do not need to know about each other. This is purely for sharing the internet only. What am I missing to be able to achieve this?

Here is my ghetto diagram of the current setup: https://imgur.com/a/FF7aqWv