On the bottom of Network Setup, you should see a listing of IP's that are leased. Find the one for your printer that keeps changing IP's, click on "RESERVE" and change it to whatever you like or leave it where it is. No more auto DHCP, it's now static.
It does have to be an IP within the range of IP's that your router is configured to assign currently, ex. 192.168.0.100 - 192.168.0.199 as long as it does not conflict with any other currently assigned address.... that is if you do decide to change it...
Some devices request their last known IP addresses and get them if available, especially if your router is tracking connections. If everything is running on auto DHCP and you are shutting everything on and off, its possible that EVERYTHING maybe getting different addresses.