I have this problem with the archer where I got a gb connection but somehow the wan port keeps getting capped at 100mbps and at first when i clicked auto negotiation and back to full duplex it switched but that does not work anymore as well., somehow I got a couple of hours a gigabit network but it magically switching back to 100mbs and this keeps poppin up in the logs:

47 INFO 0days, 00:05:13, [wan]advanced ddns -wanChanged

48 INFO 0days, 00:05:13, [wan]systool sntpc -sntpRequest

I have DoH enabled with google as dns server and enabled nat / nat boost at one point I thought it was the ddns but logged in with a tplink account and it did work but after a couple hours i checked again and it was back at 100mbps, im at lost if this is something with my isp or with the router because I have checked with the isp first and they told me no we do not cap anything and did not believed them at first but as soon the c80 went 1000 full duplex made me thinking. This is the status