You may find you need to reboot the router after turning off the DHCP server. That should also cause the router to forget the current DHCP existing allocation settings.
Yes manual settings are only offered if the unit can’t get a DHCP address and there is no other way of getting to them. I believe they prefer DHCP to avoid manually caused address conflicts.
The address conflict may be the cause of the problem – can you not turn off whatever has the address while you do this?
I presume it is the DNS setting you want to change? If not an address reservation on the router for the HB’s MAC address would be a better option.