C6300 Cannot change MTU size

Modem: C6300

Firmware: V2.01.26

ISP: Spectrum

 

Problem: Happens when I go into NetGear Genie (via either IE 11 or MS Edge) > Setup > WAN Setup.  When I modify the MTU Size (in bytes) and click Apply at the top of the WAN Setup screen I get:

 

Failure

------------------------------------------------------------

Invalid DMZ IP address

------------------------------------------------------------

Click OK to go back to previous screen...

 

I click OK. It goes back to WAN Setup screen. MTU Size has not saved, but is still the same value as before. Note that I did not attempt to change the Default DMZ Server... I left the box unchecked.

 

FWIW - NO BOXES on the WAN Setup screen are checked.

 

My question is: What do I have to do to save the MTU change?

 

Thank you  :)