Windows 10 will sometimes disconnect from a WiFi access point if it finds that internet access is not present or drops. This can be prevented if you catch the remain-connected dialog box.
It's also worth trusting the network and turning off random MAC to give the best chance of rapid reconnections. Typically if the connection is dropped, it should try to re-establish and usually succeeds quite quickly. You can turn WiFi off for a few seconds and back on again to try to get it to behave. I sometimes find I need to reboot the laptop if it fails to reconnect to WiFi when I know the access point is fine.
Some mobile router devices allow direct usb tethering to a laptop (others by RJ45 Cat5/6 cable). This would be the best way to rule out the mobile router dropping its internet connection and point a finger at the WiFi access point functionality.
The best way to confirm a fault is to reproduce it with another device such as a mobile phone. If the WiFi connection to both laptop and phone drop at the same time and both have trouble reconnecting, it would be good evidence of a fault.
Lots of other ways to go about things such as moving SIM to a phone and tethering from there to see if connection is stable (points the finger at Mifi if yes). If you are within the warranty period with the service provider, it would be worth gathering evidence of a fault with a view to returning the device for professional fault finding. I'm unsure whether any replacement device would be provided while that takes place so it could be very inconvenient.
I guess as a sanity check, you could move the devices further apart. I have a Roku streaming stick 25 cm from a Devolo access point so proximity should be no problem but I can imagine few users have their device so near the access point (so less well tested and higher chance of weird things).
