"the only decent provider in the area is a microwave
provider with download speeds of less than 15 mbps."
Changing your router will be of no use if your choices of
internet providers do not supply higher speeds (typically if
available will cost more).
It is like you currently have a 1/2 inch pipe and replace it with a 4 inch
pipe. If it is fed by an eyedropper, you will still only get eyedropper
amounts.
So, in your case, if your provider delivers 15 megabit/second, it doesn't
matter if the router is 100 megabit/second or 10 gigabit/second, you
still only get 15 megabit.