It is best to set up Alexa devices on 2.4G for a few reasons:
-- Better range (which is a problem for you now)
-- It is an audio device with low bandwidth requirements, so 5G is overkill and with shorter range
-- Reserve 5G for high bandwidth requirements like streaming video devices
If still having range issues consider getting a range extender
There is no such thing as a "best router" as there are many excellent routers today, and reviews can help you find one. Maybe you do not need a new router; maybe your home could use a range extender, or better router placement in a central location, or more than bottom line ISP bandwidth, or adjustments to settings. Who knows. There are lots of things to consider.