This makes me wonder if you did not have a conflicting IP address on your network that was coming in and associating on the IP you had configured for RFX9400.

Also, just because you put the RFX9400 to a fixed IP setting, I have to also ask when you were in this configuration did you also setup PEP (V1 or V2) to specify a fixed IP address for your extender? I have used RFX9400 and RFX9600 in Fixed IP mode for years (extender configured for fixed mode) and also make sure that I specify the ip address in the extender settings of PEP as Fixed.