Well, I've had three of them, the RT-AX68U, the RT-AX86U Pro (as I had to have a router ASAP when the RT-AX68U was having issues) and now a GT-AX6000 and none of them have had issues with either radio.
As WiFi transmitters are analog parts, they're not all identical and you'll find that two identical routers or any other kind of WiFi radio, will behave differently. Wireless signal transmission isn't an exact science and this is why no-one on this planet can guarantee the same performance between two routers of the same type. Routers are individually calibrated during production and they have a target they have to meet, but sometimes something goes wrong and this target might show up as met during the testing, but once it arrives in your home, it doesn't work as intended.
As such, send it back, test the replacement unit and if that still doesn't work, well, then you either have some really weird interference or really bad luck.
As you can see below, the 2.4 GHz network is working just fine and I have an Intel AX210 card in my PC. I also don't have any devices falling off the 2.4 GHz radio.
I agree the transmitters are not all identical. Neither is the user requirement.
If it's interference, it must be pretty sophisticated, as my 2.4 GHz ESP devices maintain a consistent connection as soon as I reverted from the Asus router to the Vodafone router.
In fact, on the first full day back on the Vodafone router, they uploaded 6961 messages and on the second full day, 6960. Very consistent.
The daily numbers were all over the place with the Asus. Including zero messages on one day, after it dropped all the connections and didn't reconnect. Hardly ever at 100%.
As for the user requirements, well I'm not sure I'd notice how the 2.4 GHz band performed if I didn't have the ESP devices. I prefer my phone to connect on 5 GHz. If I connected to 2.4 GHz and it dropped, the phone would switch to 5 GHz & I might not notice.
There are numerous threads & other customer reviews referring to issues with 2.4GHz connections on Asus devices, so I'm not alone. Look through the 1 star Amazon reviews -quite a few refer to specific 2.4 GHz band issues. Very few refer to specific 5 GHz issues.
As for power output, mentioned by other contributors. Well, I still get quite respectable speeds on 5 GHz WiFi at the other end of my 4 bedroom house. So European power outputs are adequate for my needs.
The power output cannot be affecting 3 of my ESP devices on 2.4 GHz, as all are now very close to the router. The 4th is a further away, but that's the one where a stable connection is not quite as important, as that device fulfils it's intended function with or without wifi. Right now, even that is maintaining a connection to the Vodafone router.
The Vodafone router stays, whilst I think about what next.
Either I try to find another decent router brand, risk another Asus, making sure it goes straight back within the 30 day window if not performing, or perhaps tolerate the Vodafone router for a little longer.