@davidgs As mentioned on another recent thread wrt this device
No direct hands on experience with this version. A few thoughts though:-
When testing inside did you use the original PoE adapter or a replacement? 40+ years experience suggestes 'assume nothing"! 
IIRC the TTOG was supplied with a Microchip (was MicroSemi) PD-9001GR 802.3at external adaptor (backwards compatible with 802.3af). This I have seen used elsewhere. A client used 4 on a small deployment and after ~12mo (just after warranty ended!) found one had a soft failure. When checking device was ok on light load but drooped on higher load. Couldnt be certain but he noticed spec limits external (ambient/case) temp at higher loads, but was more relaxed at lighter loads. The one that failed had been mounted in an orangery/conservatory/greenhouse like structure and in summer got real chuffing hot! (failed late summer/fall period). Perhaps check your OG unit with alternate feed? Think direct replacements are ~£40-50 + VAT online.
Another possibility is a close proximity lightning strike (not direct hit of course!) inducing a soft fail, that on power cycling turned hard? That may impact either end (in GW or on PoE adaptor) - was that a possibility?
Unit is actually a Browan OEM design/build so suggest perhaps try their tech support for assist - may get schematic for details GW fault tracing or even looking for alternate power injection points? I would be with Nick on the Caps as likely, followed by protection diode(s) or inductors…