Foot thick stone walls - that really is the problem and changing the router will not solve it. You might get a slight improvement, however it will still be a problem. I see exactly te same problem on a house in Scotland.
You really need to get some wireless access points (WAPs) and cable those back to a small switch which is connected to your existing router. Install a couple either at ceiling level n the ground floor or floor level upstairs and you should get coverage on both levels.
This will allow you to do a lot more research on what you are going to use when FTTC arrives and you will be able to use the WAPs at that time too.
M H C
taurus excreta cerebrum vincit