It would appear that either your Internet backhaul or the TTN infrastructure is not operating quickly enough to meet LoRaWAN timing requirements.
This message is a little bit confusing, because by “too much in advance” it really means “too late”. The microsecond counter against which packets are timed rolls over at 32 bits, so values in the recent past are treated as being absurdly early for when that count would again be reached in the future.
In this case, in response to an uplink packet ending at:
25818521 uS
The gateway is being asked to transmit exactly 1 second (1000000 uS) later at:
26818521 uS
But it’s not receiving or processing that request until
26836065 uS
Which is to say about 18 milliseconds too late - arguably “just missed” but really you want to be seeing things getting there hundreds of milliseconds in advance of need.
Allowing for routing delay through infrastructure of growing complexity is part of why TTN appears to be moving to a 5-second RX window in their newer V3 setup. Perhaps you’d do better registering your gateway with the new TTN version rather than the old one which it sounds like they are going to be phasing out anyway.