What's your router model?
A select few allow for external antennas to be attached, which are often a lot more powerful that those built into the the router.
Also, the router antennas are almost always omnidirectional, meaning a lot of the power is used up in sending a signal in the opposite direction to what you want. You can sometimes replace the antenna with one that isn't omnidirectional, meaning you get more of the power directed where you need it.
One of the easiest ways would be to add a repeater to your network, positioned between the two points you want to have communicate. The reapeater basically relays the signal it receives.
By the way, welcome to the forum!