When I look at the no-ip.com dashboard, I can see that the target is correctly showing up as my router's public IP (in the form 100.100.xxx.xx, which matches the IP I see in the router config page).

100.100.x.x is unfortunately not a public IP address. It's from the private address range reserved for CGNAT use – i.e. above your router there's another layer of NAT done at ISP level.

In other words, "no static IP" doesn't necessarily mean that the alternative is a dynamic IP address; it could also mean that you don't have a public IP address at all.

Looking for your IP at your router's settings page was the correct thing to do, but you should've also compared it with the IP that websites report. In your case they'll differ, indicating CGNAT.

Port forwarding will not work in this situation, as it would also need to be configured on the CGNAT level at the ISP. Use a VPN tunnel for inbound connections instead. (There are VPN programs more suited for this purpose.)

When I look at the no-ip.com dashboard, I can see that the target is correctly showing up as my router's public IP (in the form 100.100.xxx.xx, which matches the IP I see in the router config page).

100.100.x.x is unfortunately not a public IP address. It's from the private address range reserved for CGNAT use – i.e. above your router there's another layer of NAT done at ISP level.

In other words, "no static IP" doesn't necessarily mean that the alternative is a dynamic IP address; it could also mean that you don't have a public IP address at all.

Looking for your IP at your router's settings page was the correct thing to do, but you should've also compared it with the IP that websites report. In your case they'll differ, indicating CGNAT.

Port forwarding will not work in this situation, as it would also need to be configured on the CGNAT level at the ISP. Use a VPN tunnel for inbound connections instead. (There are VPN programs more suited for this purpose.)