WiFi connection Unstable

I've resetted the router and the problem persisted for some time. Suddenly it stopped and hasn't returned.

I do still have an issue with the WiFi bandwidth. It seems to run very slow. In the settings I've checked that both are set on max (600 Mbps 100% on 2.4 GHz, and 2400 Mbps 100% on 5 GHz). Guest WiFi is disabled.

With an internal network speedtest (iperf to a local wired PC), I only get around 80 Mbps.

iperf3.exe -c 192.168.1.3
Connecting to host 192.168.1.3, port 5201
[  4] local 192.168.1.8 port 51877 connected to 192.168.1.3 port 5201
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-1.01   sec  6.75 MBytes  56.3 Mbits/sec
[  4]   1.01-2.01   sec  13.6 MBytes   114 Mbits/sec
[  4]   2.01-3.00   sec  9.00 MBytes  75.8 Mbits/sec
[  4]   3.00-4.00   sec  11.0 MBytes  92.3 Mbits/sec
[  4]   4.00-5.00   sec  5.38 MBytes  45.1 Mbits/sec
[  4]   5.00-6.00   sec  11.5 MBytes  96.1 Mbits/sec
[  4]   6.00-7.01   sec  10.1 MBytes  84.7 Mbits/sec
[  4]   7.01-8.01   sec  9.88 MBytes  82.5 Mbits/sec
[  4]   8.01-9.00   sec  8.50 MBytes  72.1 Mbits/sec
[  4]   9.00-10.00  sec  12.2 MBytes   103 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-10.00  sec  98.0 MBytes  82.2 Mbits/sec                  sender
[  4]   0.00-10.00  sec  98.0 MBytes  82.2 Mbits/sec                  receiver

iperf Done.

Note that 192.168.1.3 is a cat6 connected PC.

I should get at least 550 which I had on my previous low-end router. I've enabled/disabled some WiFi settings (AX, Smart Connect, 20/40 Coexistence) but nothings seems give a decent WiFi speed for this "high-end" router.

Should I return it for warranty? I've made this speedtest short after I've resetted the router (minimal settings adjustments done).