Can we try to keep things on topic to what the OP’s trying to get help with?
I've removed my post as it was not helpful.
Not sure if I've seen this mentioned yet, so just in case: model C-5 and later hubs let you set a static IP address on the hub under Settings > Networking. However, if you don't have control of DHCP on your network, I'd recommend against this--you'd need to choose an address that DHCP would never allocate, so you'd need to be able to know where that is, and if you're capable of controlling that on your network, you should also be able to just reserve an address for the hub there instead (ultimately a similar result, just less to worry about if you ever move the hub to another network--you don't need to reset it to gain access). I see lots of people are trying to help you figure this out with your network above, and I don't have anything more to add to that part of this discussion.
A separate issue is that your mobile app or whatever you're using to access the Dashboard doesn't seem to be getting the new IP address. That sounds like a problem to me, and I'm not sure any of us know the details of how the app finds it, but my best guess is that if it has Internet access, it tries to retrieve it from your Hubitat cloud/portal account that the hub should periodically check in with. I sometimes move my test hub to another house/network and see it update pretty fast there, though I don't use this one with the app so don't really know. If you're using a browser and only have one hub, hubitat.local
might work better than the IP address. This part we have less control over, but if your hub has Internet access, it seems odd that it's not happening...
Just curious what model wifi extender are you using? I guess you are using the extender in client mode (ie as a bridge) to connect the hub to the main router?
I am using it in browser from local address lets say 10.0.0.15 . When the address changes to for example 10.0.0.20 all subpages are also on this last address with exception of the dashboard pages. Hub still looks for them at 10.0.0.15. I think this should be easy to reproduce.
Rebooting the hub helps.