Cannot change IPv4 Subnet of Router

I have OpenWRT installed on a Netgear R7800 Router, where I have one interface configured for Wifi Clients. Unfortunately I'm not able to change the subnet IP. By Default it is, I want something like Every time I change the settings on this page, it just tries to apply the changes and then reverts them again after 90 seconds: image

It used to work before, I don't know why it doesn't work now. Any ideas what I can do? I have so set up two subnets, one for admin access and one for guests.

Apply Unchecked instead of Apply