RS700 WIFI coverage issues - extender?

One of the reasons might be the band you're connecting to. Wifi 7 and the RS700 in particular, use smart connect. It combines all the wireless bands into a single ssid. And then your device roams between the 2.4/5/6ghz as needed based off its own protocol. If you're connected to the 2.4ghz network, its a much slower network and sensitive to interference. The 5/6ghz networks are much much faster but dont' have the same coverage and pentration powers. So the slower speed might have been the 2.4g connection and then the device roamed to the faster band. 

Tough to know 100% without checking it right after the test to see which you were connected to. One thing you can do, is disable smart connect (temporarily), connect the phone to the 5ghz, and walk around the house. Test different areas and keep track of the speed. If you're getting fine speeds on the 5ghz, its more than likely just what band the device was connected to. 

The band roaming is a good thing as it allows devices that need higher bandwidth to move to the band needed while slower devices like IoT devices, stick on the slower 2.4ghz network. But it does take a bit for the network to "settle down" and for devices to figure out what band to be on.