@RouterRob mine is doing this too the last few days can't find any reason for it either! 

@jefem24 We have a lot of devices in our household, so I was trying everything I could to avoid a factory reset because I didn't want to have to reassign all the devices and all that. But I contacted support yesterday and was basically told the weekend starts on Friday and nobody would be able to help me until Monday or Tuesday, they said they needed to escalate the ticket since trouble shooting wasn't doing anything, in the meantime my router decided to start disconnecting about every hour. So I decided to reset it, this was at about 10PM last night, and now here we are at 6 PM the next evening and it hansn't done it since.

@RouterRob I'm getting the same thing for the past week.  Orange light comes on the router and a reboot fixes it and I have a new DHCP IP. I have an Arris Surfboard in front of this and logs in there show: 

Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=20:3d:66:9f:aa:8f;CMTS-MAC=00:01:5c:67:20:75;CM-QOS=1.1;CM-VER=3.0;

SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=20:3d:66:9f:aa:8f;CMTS-MAC=00:01:5c:67:20:75;CM-QOS=1.1;CM-VER=3.0;

Spectrum is my ISP and a call with them went nowhere, but that was before I had this log info.  I'm going to get back on the phone with them Monday.  This all started happening about a week ago and after it did, there was a service outage that night for about 3-4 hours.  I feel that the problem is between the Surfboard and Spectrum, not the TP-Link AX21.

@RouterRob I just bought a TP-LINK AX21 (hardware version 1.26) for use as a router with Ziply Fiber and have been having the same issue - Internet WAN connection goes down around every 4 hours. I had immediately upgraded the firmware to the latest version it autodetected (same as yours). I've had to reboot it multiple times, and it's been in service less than 24 hours. Frustrating.

I just installed the beta firmware version mentioned here: https://community.tp-link.com/en/home/forum/topic/521268

The article is intended to address: "Some customers report that the router keeps dropping from the internet/modem since 1/7/2022, some mentioned this happens after they upgrade the router firmware to the latest 210901". So seems promising.

Let's see how it goes.
 

@mi5key Well at least you have log info to go off of. Everytime I reboot my log says that data was cleared because time changed. My modem is a Netgear nighthawk cm1200 but it never loses internet connection (isp is Comcast) so it seems to be a problem specifically with the router. I did a factory reset at 10PM on Friday and haven't had any issues since, prior to that it got to where it was DC'ing about every hour. I'm still waiting to here back from TP-Link support to find out what may have caused the issues, Incase it arises again.

@djmess Don't know how severe your issue is, as stated in other replies mine started to DC about every hour so I had no choice but to factory reset. That was Friday night, I haven't had any issues since so I'm hoping it was a permanent fix. TP-Link support is supposed to get in touch beginning of next week so maybe they will have some info on whatay have caused the issue.

@djmess This is the exact issue I'm having.  I'm going to install this beta and give it a go.

@RouterRob  What kind of router do you have? I've got an AX90 and this just started happening to me between last night and this morning – the router loses connection every hour, almost to the minute that I last restarted it. You say a factory reset seems to have taken care of the issue? I've also just now updated to the latest firmware. I last restarted at about 11:15am EST so we'll see what happens in 1/2 an hour or so.

EDIT: Exactly one hour after my last restart, the router lost connection again. I've done a factory reset. We'll see how that goes. Crossing my fingers!

@djmess FWIW, this beta firmware update seems to have solved the issue for me - so far, no more disconnects observed.

@RBWakefield I have the AX21 1800. We are now in day 3 since the factory reset and still no issues. So hopefully it will take care of yours as well.