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Hmm from what I read if I understand you correctly:

You got a modem using the PPPoE connection correct? (I assume this because its often used as a isp connection type), this modem is the SF1200?

And your router (missing name here) is behind it using the wireguard client? And the server is on the SF1200?

I think you have to prototype a little here first without the wireguard because it might make things doable.

So the router has a NAT, and the modem has a NAT, with other words your router would be aware of your computer but on the part of the modem, the modem can only see the router as a client and not see the client list like inside the router.

Then there are two options:

A)
You have to make the router static on the modem side, then portforward the input rules for this port and forward it to this router ip.

Then on the router you also portforward but then you forward it to your pc (this ip needs also static), and then of course have the wireguard share option enabled.

If im right but I’m not sure how the ddns works your network should be reachable.

B)
Just my speculation, but wouldn’t it be easier to use a tagged vlan and make it so that the modem handles the dhcp, firewall and maybe even wireguard so you won’t have to deal with the double nat issue?, I had a situation with three nats but I needed a way to reach the clients over one cable, vlans could be very powerfull to traverse into a network topology it might be a good idea depending on your complexity of the network.