Internet (not wifi) connection dropping for only one device on my network

Hi all

Looking for some help.

My set-up is as follows. Airport Extreme connected to DSL router (Fibe in Toronto). I have an Airport Express acting as a dedicated network extender.

All had worked pretty well overall for the last 2 years.

Last weekend a Bell technician came and installed a more modern router. I did the same set-up as before, I disabled that router's wireless function and connected it to Airport Extreme.

The performance when it's working is very good.

But on my laptop on my 3rd floor office I am getting 'no internet connection' errors with the exclamation mark symbol in the desktop wifi icon. At first I though it was an issue with the Fibe router. But then I noticed that my iPhone's internet on the same network was functioning properly, as were AppleTV, iPad, other devices. The only thing that fixes it for my laptop is to go in to Airport Utility and restart Airport Express. But I've been having to do this 3 times a day.

Does anyone know what could be causing this and what I could do to fix it? Why would Airport continue to serve other devices no problem, but stop serving internet connection to just my laptop (it's a Retina MBP and I've been using it for a year with no connection issues until we changed the DSL modem last weekend.

I'm thinking it has something to do with DHCP settings/leases but I'm not sure....

Any help is greatly appreciated.

Thanks!