GL-AXT1800 (Slate AX) connects only with 866 Mbit/s

I have already some routers from GL-iNet and this one Slate AX is new. I have a Fritzbox 7590 which works fine and some Fritz Repeater (AVM). The repeater connect with 1300 Mbits and I can get 1 Gigabit in the WLAN. But Slate AX connects only with 866 Mbits and provides only 866 Mbits. In the Fritzbox the speed is displayed with 866 of 866 Mbits. In Luci its also displayed with 866 Mbits (866.7 Mbit/s, 80 MHz, VHT-MCS 9, VHT-NSS 2, Short GI). But it should provide 1200 Mbits. I could not change it.
I updated the firmware. I resetted the settings. I tried as router (connected to Fritzbox WiFi). It tried in bridge mode. I put the Slate AX directly to the Fritzbox. Fritzbox uses 5 Ghz, WiFi 5. AC.
Can you help?

I tried switching the AXT1800 as a repeater,and the repeater connect with 1200 Mbits,the superior uses WiFi 6.AX:

you should make sure that the repeater and the superior both use WiFi 6, and What day’s firmware you’re using?

my router supports Wifi 5. Could it be that GL-AXT1800 only supports up to 866 Mbit/s with a Wifi 5 master?

Yes, the connection speed is negotiated between the two devices, if you wan the repeater to connect with 1200 Mbit/s, it must be connect with a Wifi 6 master.