AVM Repeater 3000 as AP (WIRELESS)

I just realized the 5 GHz radio is QCA9984

A quick Forum search for AVM Repeater 3000 found other person with similar problems as you in 2020:

When I went to the firmware selector, there was no master snapshot available for this device, so in the meanwhile, maybe there were problems and it has been dropped for whatever reason?

If you are unwilling to dive into the configurations, the only other thing I can suggest for you to try an older version of OpenWrt. The AVM Repeater 3000 was added in version 2019, so OpenWrt version 19.07 (not supported anymore!) or 21.02 should have been the one that hopefully worked properly. I have not found any information in the commits that would mention 5 GHz channels scanning not working, but sometimes things regress and newer versions of OpenWrt break functionality that was already working in older ones. Maybe this is what you experience.

Hope you have set your country code!

The AVM has three radios (2 x 5 GHz - QCA9984 and IPQ4019, 1 x 2.4 GHz - IPQ4019), are you sure you have configured them correctly? One of the two 5 GHz radios usually would be used for the wireless backlink to your router, and the other one provides the access point for your clients. I don't think there is any sort of "repeater auto-configuration" with OpenWRT (as the AVM firmware for their Fritz! products has).

The different 5GHZ networks can already be recognized by the SSID (Channels) found.
That is clear.
Surprisingly nothing is found with the normal 5GHZ scan. You don't have to set anything there.
Oddly enough, channels over 100 are found in the second 5GHZ network.
I then tried to set up an access point via this second 5GHZ network, just like the 2.4GHz before.
Then the entire network crashed, so I ended the Openwrt topic.
To be honest, this software is not for me as an absolute beginner.

Well, there you have it. The QCA9984 (radio0) is limited to channels 100+, IPQ4019 802.11acn (radio2) to channels 36 to 64. That's a hardware limitation, likely to prevent interference (and maybe even a regulatory requirement so that there are not two radios on the same frequency in very close proximity),

What I don't quite understand is what you were doing for your wireless backlink. Unless you connect back to your router and bridge the networks appropriately, your secondary SSIDs won't route anywhere but between clients connected to the repeater.

That's why I say that the software overwhelms me.
I connected to my 2.4GHZ via WLAN as a client. Then set up an access point (master).
And I wanted to do the same with the 5GHZ.
It can't really be that difficult with a GUI. But apparently it is.
There are now 5 clients connected to the repeater and I'll leave it at that. I had previously imported a backup. And now it stays the way it is.

If your using the 2.4 GHz radio as the uplink, your effective speed will obviously be limited to that when accessing anything but other clients connected to the repeater. It's not intended to be used that way. You should use one of the 5 GHz radios as the uplink (and only as the uplink).

How about the instructions for dummies?

I would like the repeater to pick up its clients via the 2.4 network and the 5GHZ network.
And with the same SSID as the router, so that all WiFi devices can choose the best router.

My Guide:

I have no personal experience with a relayd setup, I use 802.11s and batman-adv (which is certainly a bit more complicated, although I found this guide very helpful if you want to dive in). However, you will always have to set up your SSIDs yourself, there is no way to just have the repeater "copy" them from your router.

Anyway, since I think you never stated this anywhere: Is your router running OpenWrt as well? If not, why do you want to use OpenWrt on the repeater?