No quantity of hard resets with manual reconfiguration will fix this with current firmware. The problem is reportedly that the current GPL wireless driver lacks support needed to interrogate temperatures on the newer Broadcom chip, which is used in hardware rev A5 of AC-3100 / RT-N88U. The wl command returns "unsupported" when asked for radio temps, thus the web interface shows "disabled".
I've hard reset (both via red reset button, and via power-on with wps button), and manually reconfigured my RT-N88U multiple times -- radios still show "disabled" with 380-65. After a hard reset, even before any reconfiguration, the radio temps both show "disabled". It's because the current wireless driver can't read them from A5 hardware revision.
Hopefully it will be corrected in a future GPL release.