I tried this morning, same symptoms.
Setup :
Weird part is, the PC remains connected seen from the acces-point.
Test pending : I have disabled radio0 (5GHz) and am trying on radio1 (2.4GHz) to see if the same occurs.
I will report the result in about one hour.
Edit : the same happens on 2.4 GHz 
Trying with 5GHz back on channel 116/160MHz but with WPA2 PSK this time (no radius). Will report in about one hour.
Edit : using WPA2/PSK did not cause the hourly disconnection. It seems the problem is in relation with WPA2 Enterprise. After some searches, I found this thread, mentioning rekeying problem.
I will try with different values for parameter wpa_pair_rekey to see what happens.
Edit : setting wpa_pair_rekey to 30 seconds, I can reproduce the problem : disconnection occurs after 30 seconds following the correct authentication.
I have set this parameter to 0 and will see what happens...
Edit : 0 for this parameter was a mess : connecting and disconnecting tight after. Trying with 86400 seconds to see...
Final tests : bad luck, despite the values of the number I choose (7200, 86400, 9), I get disconnected after one hour 
It seems the problem is strongly related to WPA2-EAP, on my other router (WRT1200AC) it works well since years.
Any idea on what other parameter I could investigate ?