I can only find information on blocking cifs, not allowing it.
I have a RB750 that has a few static routes on it to allow traffic to go to the next relevant router (ubiquiti radios)
Where traffic is statically routed by the ubiquiti radios to the next ubiquiti radio, windows network sharing works.

But when the traffic needs to pass through the mikrotik routes, the traffic is blocked, I can tracert through the mikrotik - but not access shares "on the other side" of the mikrotik.

I've read that I need to enable TCP port 445, amongst 7 others. Is nat involved? What is the firewall rule I need?

Internet firewalls can prevent browsing and file sharing

The following ports are associated with file sharing and server message block (SMB) communications:

Microsoft file sharing SMB: User Datagram Protocol (UDP) ports from 135 through 139 and Transmission Control Protocol (TCP) ports from 135 through 139.
Direct-hosted SMB traffic without a network basic input/output system (NetBIOS): port 445 (TCP and UPD).

Also read this article: Service overview and network port requirements for the Windows Server system