To see/confirm that the device has exceeded the limits of a 16 bits uplink frame count (65,536), one can paste a Base-64 encoded payload from the gateway’s Traffic page into an online decoder, and then also specify the AppSKey and NwkSKey. Using those secrets, the decoder will show the MSB of the frame count (which is not in the packet itself, but needs to be determined by the server, for which a real LoRaWAN server then assumes the counters are not too far out of sync).
See After resetting frame counters, payload is shown on gateway traffic but not in application to re-sync the uplink frame count in TTN and the device. That won’t work for the downlink counter though, so simply restarting the devices is surely easier, but maybe you want to know what’s happened.