@GRL Thanks a lot for quick response. Based on your answer I was able to find the channel selection in Wireless Settings. Recording the solution here to help possible future readers with the same issue: The problem was that "Smart Connect" is by default on. If you uncheck this field, additional UI fields with properties for 2.4GHz and 5Ghz bands will appear on that page.

I have two follow-up questions:

1. Is there any drawback in disabling Smart Connect feature? According to description it seems the router should balance clients between 2.4 and 5GHz bands if it's enabled. Will disabling mean the performance could be worse than when it's enabled? Or do I get it wrong and this feature basically just automatically sets the properties that I can set manually when it's disabled? In that case setting them manually to reasonable values would be identical (or better) than keeping it on, right?

2. It seems you are suggesting that in AP mode the MAC filtering is performed by the upstream router. Is that so? I thought MAC filtering is used for wifi only. If the client is connected through AP, the upstream router sees it as connecting through regular RJ45 port and no MAC filtering is performed, just like for other clients connected to other RJ45 ports.

@GRL Sorry, but apparently this is not how it works. The upstream router will know the MAC address, but it will not apply MAC filtering, since the connection doesn't arrive through wireless, but through one of wired ports from its perspective (wireless-wired conversion is done at the AP). So the MAC filtering needs to be done by the AP itself when communicating wirelessly with the client.

To confirm, I've made a simple test - I removed my phone MAC address from list of allowed MAC addresses at the upstream router. As expected, I'm not able to connect to my upstream router using its wifi network, but I'm still perfectly fine connecting to the internet through my Archer AX 53 and the MAC filtering denying the connection on the upstream router doesn't apply.

@Honza987 So just for record if someone stumbles upon the same issue as I did. There is no MAC filtering, but there is a similar function called "Access Control" which can be used as well. It's in Advanced / Security / Access Control. The caveat is that it's  not accessible in AP mode. So I had to reconfigure the router back to router mode and do through "manual router to AP" process (disabled DHCP server, set IP address and reconnect uplink from WAN port to one of LAN ports). Hope it helps.