I know 10 days ago but still gonna answare.
I’m no expert, but even though when you use the same IP table in the main router and the mini router there are two different networks.
I tried it my self and it is not possible to access the mini router network when you are in the main router network. You need to connect the mini router and the main router with the LAN port (on the mini router) but then you can’t connect to an VPN in the mini router. If you connect it that way devices even connected to the mini router network will be in the main router network because the mini router does not open an network. I think kinda confusing explained. Look at this tread : New user help using lan port for vpn - #10 by JonnyJaap
You can forward the port for SMB in the mini router (see also the other tread).