STA + AP Configuration and speed issues

I just bought a second hand Asus rt-ac51u, since it was supported by OpenWrt and had a USB port.

Installation went fine and I got it running easily. However STA + AP configuration has been much more demanding. I have it now running, but download is quite slow through wlan.

I have set one radio to work only in client mode (5 GHz) and the other only as a master (2.4 GHz). I share mobile data from my cell phone (4G/LTE, 150 mbs limited).

It works just fine. I get internet access from lan and wlan.

The problem is speed. If I test using Speedtest in the net (or app) I get about 50 mbs download and 25 mbs upload from directly from the phone or from a PC connected to lan port. But if I connect the PC to 2.4 GHz, I get about 3 Mbs download and 25 mbs upload.

I get the same 50/25 mbs also from OpenWrt using speedtest-netperf script.

Th 2.4 GHz wlan seems to be working OK between OpenWrt and PC. I can transfer a 25 MB file in 10 s in both directions. So there must be something else limiting the performance. Top doesn't show high CPU usage.

So what is causing this slowness?

Also it would be nice to have both radios on 2.4 GHz or at least both Client and a Master on 2.4 GHz. I tried to do that any many different ways. It even worked for a while, once I turned off mobile sharing the whole wireless system stopped working. No access via 5 nor 2.4 GHz and no recontact to my mobile.

I also couldnĀ“t get the other radio to work at 2.4 GHz. I can choose 2.4 GHz, but it wont work as a Client nor a Master. 5 GHz it works OK.

STA + AP should be possible on the same radio, but only one channel/radio is available.

5 GHz has very limited range here due to brick walls. That's why I would prefer only 2.4 GHz, which is not at all crowded here, just one station from the neighbor can be seen.

Please test more 2.4GHz devices(use some phones), maybe your PC wifi problem.

I tried with my old phone (Samsung S8). I got 50-60 mbs download while connected to the WiFi shared by my current phone (S20FE). Both 2.4 and 5 GHz.

But connecting S8 to OpenWRT 2.4 GHz showed a bit less than 3 mbs download. Upload was about the same 20 mbs, which I got using S20FE or S8 connected via WiFi to S20FE.

So there seems to be something slowing down the bridge between the radios of Asus running OpenWrt. But only while downloading through OpenWrt radios.

Download speed always starts well and then drops when I run from PC connected to OpenWrt with wlan. It starts faster (e.g. 17 Mbs for a few seconds) and then drops all the rest of the measuring time (e.g. 5 Mbs after 5 s and 4 Mbs at the end). While upload works normally. Increases in a few seconds to 25 Mbs and stays there all the rest of the time.

I tried setting up both STA and AP on the same radio (5 GHz). It was fast, I got 40/27 Mbs from PC connected to OpenWrt with wlan. But the problem is very short range here due to brick walls.

So the problem seems to be clearly related to the data transfer from radio to radio. Is there something wrong with my setup? How can I find out what causes the problem?