Hi,
as a preface; I know pretty much nothing about networking. I needed a new router and I was recommended hAP ac² which I bought.

The box should have default configs except that i changed wlan passwords and enabled 2.4Ghz guest network.

I have 1Gbps connection but I get only 20Mbps though 5Ghz wlan with my new Debian laptop. My phone gets around 100Mbps.

I don't know if it is related but I cannot get Windows laptop to connect to the guest network. It connects, immediately disconnects and says that it can't connect to this network without further explanation. I disabled EnableActiveProbing from registry HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NlaSvc\Parameters\Internet as instructed in this thread but it did not solve the issue: viewtopic.php?p=1009431

But anyways; the download speeds are the problem, I just figured that there is a chance that these are connected.