How to properly set up a VPN server?

Question

Mikrotik

How to access the Internet according to the following scheme:
1. There is PC0 connected to the router with the OpeenWRT firmware
2. If the balance is positive, the host PC0 has access to the Internet through the “router in the house”
3. There is PC1 connected to the router with the MikrotikOS firmware
4. When a negative balance of PC0 Internet access is limited BUT at the same time, the OpenWRT router has access to the router with MikrotikOS.
Actually the question is HOW to correctly configure the VPN server on Mikrotik so that when connected from the OpenWRT network, you can access the Internet
PS the conditional diagram is attached