There may be several reasons why that is happening such as WiFi interference. It may also be smart home devices, baby monitors, WiFi security cameras or WiFi speakers causing interference. Just to confirm, is your Shaw modem currently bridged to use the 3rd party router or is an IP passthrough set up to use both?
To start, I'd recommend removing the router from the setup. If the 2.4G network disappears again, then look for 3rd party equipment in your home and start turning them off to see if the WiFi connection remains stable. Keep me posted on how it goes!
Tony | Community Mod.