I'm behind a large ISP CGNAT and I requested a block of 4 public IP addresses for my connection. They sent me back a block of 4 public IPs with only the two IP addresses in the middle as usable (below is an example not using the real IPs)
100.64.0.3, 100.64.0.4, 100.64.0.5, 100.64.0.6
They also sent me some updated static IP settings for the router consisting of private IP space:
WAN IP: 10.20.20.21/29
I've set the static IP setings for the router but when I check my public IP using whatismyip, it doesn't return an IP in my public IP block.
I'm sure I'm missing something here, I did try the instructions on the OpenWRT guide here but in 19.0.4, Luci informed me that the configuration needed to be updated in some way and my connection stopped working.
Sounds like some kind of mapping, but you should tell us their verbal advice/docs, if they have provided you anything. Based on just those addresses it is pure guessing what they are actually providing.
Yes, that would make sense and I assumed that the mapping would happen naturally once those WAN IP settings on the router were set but it seems like there might be some additional configuration needed.