[Newbie] L2TP/IPsec port forwarding

Good day, all. Newbie in mikrotik here. I am trying to configure my mikrotik hap Ac2s each in two different offices to have an L2TP IP tunnel that connects between them as per the diagram. However, after reading online, I need to do some port/NAT forwarding shenanigans on them because the mikrotik does not directly connect to the ISP. I am a bit confused about the steps. What I am planning to do is open the necessary ports on R-ISP and the mikrotiks on both offices.

On the current configuration of the mikrotiks, all LAN ports are bridged and there is an src-nat masquerade rule with the out interface on the WAN port (ether1). If I remove that rule, the mikrotiks are unable to connect to the internet.

Then, I got stumped about the source IP and destination IP. In this field, what should I put?

  1. For the R-ISPs, should the source IP be left blank and the destination IP be the WAN interface of the mikrotiks?

  2. For the mikrotiks, should the source IP be left blank and the destination IP be the R-ISP’s LAN IP address?

  3. In order for both servers to be able to do basic requests to each other, must I open the necessary ports (HTTP, FTP, etc) for it to work? If so, must I do it both on R-ISP and mikrotiks and what are the correct source and destination IP addresses?

If these are insufficient, what are the additional steps or actions that I must take? The diagram is in the attachment to this post
Network Tunnel Diagram-Page-1.jpg