Orbi RBK352 / 353 WiFi AC performance are very bad, SOLUTIONS?

To do more thorough checks, I put two Samsung Galaxy A70 devices (WiFi 5 AC) in front of the router in 5 GHz, and ran an iPerf test. This test measures speed on the network, without an external internet server. One of the two A70 acted as a server, while the other acted as a client. The result was above 250 Mbps. Given this, it is quickly clear that the problem is not the actual performance / hardware of the router. There appears to be a “bandwidth retention” problem in the WiFi 5 AC network. If you perform a speedtest with any test with an external internet server, such as Ookla, the speed is limited to “only” 150 Mbps. Strange, similar problems are not from Orbi. I also noticed another unpleasant thing. In the Advanced section, Advanced Setup Wireless Settings, I am sorry to note that, for some reason, it is NOT possible for me to change the WiFi channels for the 5 GHz frequency. The option that should allow the choice of the channel is faded, and it is not possible for me to click on it and choose a channel manually or set the automatic choice. The router is stuck on DFS channel 108, and won't move from there. Different for the 2.4 GHz, where you can freely select the channel, or set the automatic mode (as it should be). How is it possible? Even on cheap routers it is possible to change the wireless channels for both frequencies. On the previous RBR20s this was possible for both frequencies. Finally, I would like to add that often the RBR350 router “loses” the connection with the RBS350 satellite. From the router interface, by going to the connected devices section, you can also see the satellite. In the section indicating the status, it says "configuration synchronization in progress". It should come out "good". From the Orbi app, the devices are actually connected to the router and the satellite is OFFLINE. But if I go to check, the satellite is working. The devices move correctly from the router to the satellite and vice versa. So in reality the mesh and the satellite work correctly but their status in the app or in the router interface is offline. I have a LAN, so the satellite is connected to the RBR350 router via CAT6 ethernet cable ... It is not possible for it to lose the connection (and in fact it does not actually lose it, I think it is a bug in the interface that shows the satellite as offline even if it works fully). I tried to configure the satellite wirelessly. Same result. Satellite shown offline but functional.