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Hi @VinceO . What type of NBN connection do you have? (FTTP, HFC?)
What model wifi router?
Is the router providing sufficient coverage apart from yellow area?
Where is the hardwired TV? If in living room, leave as is. If elsewhere, you could put a cabled extender there and have TV connect to it by wifi, or by cable if it has 2 ports.
You can use a wifi laptop to map out the coverage through the house. Useful if it handles both wifi bands. It shows surrounding networks and relative strength and channel number.
Use the command: netsh wlan show all
Else, use a tablet which shows the wifi connection speed to the router. Speed decreases with distance and weaker signal. 2.4 GHz band has greater distance; 5 GHz has higher speed.
One or two extenders might be sufficient.
Routers support 250 ip addresses but usually don't allow that many devices to connect. That is a consideration for a replacement router or mesh.
If you decide to go with a mesh, the specs include the area of coverage. That figure would be reduced because of the walls so an extra satellite might be needed. Getting coverage outside house might mean satellites are cascaded.