In my experience, Wi-Fi extenders don't significantly improve a low-spec router. Invest your money in a wireless router and get tri-band and good radio. I understand how the grid works in theory, although in practice I was frustrated and frankly didn't benefit much from being able to move around the house seamlessly: Important appliances are static. This Wavlink router is inexpensive to spec and gets the job done. Band 3 is a great feature as it has a stronger signal and less interference (I'm surrounded by dozens of neighboring SSIDs).
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