Mesh betwen R7800 an R6120

My problem is that this device establish a mesh connection but the throughput is near 0

openwrt based on 18.06.4

iperf -s on the R7800 and the the R6120 as iperf -c
root@R6120:~# iperf -c 10.31.45.10 -P2
------------------------------------------------------------
Client connecting to 10.31.45.10, TCP port 5001
TCP window size: 43.8 KByte (default)
------------------------------------------------------------
[  3] local 10.31.20.192 port 54178 connected with 10.31.45.10 port 5001
[  4] local 10.31.20.192 port 54180 connected with 10.31.45.10 port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec  10.0 MBytes  8.37 Mbits/sec
[  3]  0.0-10.1 sec  15.2 MBytes  12.7 Mbits/sec
[SUM]  0.0-10.1 sec  25.2 MBytes  21.1 Mbits/sec

wen i change the direction iperf -s on R6120 and iperf -c on the R7800

root@Netgear_R7800:~# iperf -c 10.31.20.192 -P2
------------------------------------------------------------
Client connecting to 10.31.20.192, TCP port 5001
TCP window size: 43.8 KByte (default)
------------------------------------------------------------
[  4] local 10.31.45.10 port 39304 connected with 10.31.20.192 port 5001
[  3] local 10.31.45.10 port 39302 connected with 10.31.20.192 port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec   112 KBytes  91.4 Kbits/sec
[  3]  0.0-10.0 sec   119 KBytes  97.1 Kbits/sec
[SUM]  0.0-10.0 sec   230 KBytes   189 Kbits/sec

on the same place a Fritzbox 4040 reach 200Mbit in both direction over the air. After i buy a other Router with MT76x2E WiFi chip an get the same problems. But the mesh networks between this tow router is MT76x2E not fast but it works. Is there an incompatibility in 802.11s between ath10k an MT76xxx WiFi ?