What kind of wireless router do you have? The kind of signal you’re putting out will determine what you need to fix it. Also, what’s the house layout and how old is it? Trying to beam through old construction is considerably harder than drywall.
There’s a couple of ways to tackle this. Simplest would be what’s called a wifi repeater or extender. That’s a unit that takes the existing signal and ‘reamps’ it, boosting the signal into another part of the building. Something like this or this (an upgraded version that handles faster signal) would probably do. The trick is to place it half way or so through the house as it has to get the signal from the router, and hope it covers the rest of it. These things are seriously YMMV, as they don’t fix a bad signal or do much in terms of penetration of dense wood/metal/etc construction.
The real way you fix this is to run a wire and put in an access point. That’s not nearly as hard as it sounds if you have a drill and a little time - drill in the corner of the room with the router, run a cable thru the ceiling to a mid point in the house, connect one end to the router and one to the access point. Couple of small holes and 30 minutes in the attic gets you whole-house coverage that you never have to worry about again.
I’ve got one of the ones I linked above that services a 3700 sqft two-story, lathe-and-plaster house (as well as out into the yard) - the coverage and penetration is great. So great that depending on your house layout and construction, you might be able to just hang it above the router and get good coverage everywhere. These things are used for wifi on college campuses everywhere, so they’re robust and easy to set up. Upgrading to one from the ‘all-in-one’ routers you get from your ISP is like driving a Cadillac after being stuck in a Camry.