@Bogbody wrote:
It should not effect the existing wifi -- unless there are no free channels.
The "extenders" that connect to the original router over wireless are necessarily using the same channel, meaning that this channel becomes more heavily used. Some mesh systems use a third radio for the meshing function, separate from those used to connect wireless devices, however a basic original ISP's home router will not have that capability. Possibly if both the router and the extender are dual band, you could make some sort of compromise by using say 2.4 to connect the two (obviously on the same channel) and 5GHz for client devices using different channels. Or vice versa of course.
Of course it goes without saying that a wired connection to a second wireless access point is the best solution to extend wireless coverage. However if that's not possible then my second choice would be power line adaptors, you can get these with the wireless AP function built into the remote unit. This is what I use to link to the garage with a TP-Link PA4010 connected to the router and a WPA4220 in the garage.
