Slow Wifi on Netgear Orbi Satellite with ethernet backhaul

Updated the firmware manually this afternoon. My firmware was 2.5.1.8 and the Orbi router dashboard said there were no firmware updates. However, by accident I saw that version 2.5.1.16 is available which was not detected by my orbi router/satellite.
Rebooted and tested the speed settings. I get 304Mb as expected. Now wait a few days and see if the speed stays stable or lowers again after some amount of time with these new firmware.

If I test the speed when I suspect an issue. I always test with 3 devices;
- a macbook pro which is connected to the satellite via wife,
- an iPhone which is connected to the satellite via wifi
- a mac mini which is connected to the satellite by ethernet.
The speed of the mac mini is always around 300Mb, also when the speed over wifi is very slow and around 45Mb. When the wifi speed is 45Mb, both my macbook and my iphone encounter these low speeds. 
If the speed is low I mostly also move my iphone to the ground floor where the router is so it can connect to the router instead of the satellite and then I always get speeds around 290-300Mb, as expected. So the issue seems to only occur when a device is connected to the satellite via wifi and the satellite is powered on for 1 or more days. Not sure after how much time the speed drops down to around 15% of the original speed.