@Piez 

I would look at the LED lights to see if all stay consistently blue to ensure the extender is located in a good position. Should the 2.4G or 5G or the circular LEd turn red, please move the device to a closer location. 

If you get the no internet connection, connect to your router to make sure network connectivity is still working there. 

What you can also try is to statically assign one of the problem devices to see if it may be something with DHCP. 

Find what your IP address scheme is.

For example, if your router gives out 192.168.0.100-200 IPs, set your device to the following static IP:

IP: 192.168.0.50

Subnet Mask: 255.255.255.0

Gateway: 192.168.0.1

DNS: 192.168.0.1