I recently updated my AX20, and since then I suffer from a lot of issues.

a couple of things that might be important:

1. I use it in OneMesh mode, in conjunction with an RE300.

2. I use Smart Connect. 
3. Wi-Fi 6 is off, encryption is WPA2-AES.
4. Non of these issues happened before the update.

The issues I have been experiencing:

1. Some devices (mainly my iPhone for some reason) randomly says the Wi-Fi has no internet connection and use cellular for a while. While that's happening, other devices continue to use the internet without any problem. 
2. Internet speeds (both wired and wireless, although wireless is significantly worse) are slower. 
3. Sometimes I go into the Tether app to prioritize a device in QoS, and when I enable the "Prioritize" switch it says QoS is disabled on the router. 
4. most of my devices seem to prefer to connect to the RE300 now, even when much close to the router and even when speed is significantly lower.
5. Binding IP address to a MAC address just completely broke. It just won't work no matter how I configure it. 
6. I'm a developer, and one of the tools I use a lot is SSH, typically over Wi-Fi. For some reason, SSH connections are ridiculous unstable, making it practically unusable. Connections either get stuck (not dropped, just stuck. I need to restart the terminal to get out of it), or times out when I try to connect to the device in the first place. The weirdest thing about this is that it's only a problem with SSH - other protocols such as VNC and SCP work perfectly fine over Wi-Fi for me.

7. it seems like now there are different IP addresses for some devices, depending on whether it is connected to the router itself or to the RE300.

8. the change log says: "Fixed the bug that the hostname is displayed as a messy code or asterisks". So my devices that were identified as an asterisk before are now identified as "UNKNOWN" - that's not a fix.

9. It takes much longer to connect to the router using the Tether app, especially after chasing a setting when sometimes the router is listed as "Unknown Router" and I can't really connect to it for a few minutes.
10. Not a bug but definitely annoying - the new menu in the Tether app for selecting a time range in parental control is really ugly and not user-friendly at all, and allows less accuracy. The older one was just fine, I have no idea why it was changed. 

Feature request:

Please allow to bind specific devices to either 2.4GHz or 5GHz when using Smart Connect, and in OneMesh allow to bind specific devices to specific access points.