You may have to relocate the router if possible, the fewer obstacles there are the better.
If the router is older, upgrading to a 802.11ac router usually works better.
Ultimately, with longer distances it may be necessary to setup a wireless point-to-point bridge, however that requires some setup/knowledge/money.
To start with the simpler solutions, I'd look into what can be improved about the current router as far as location/obstacles between it and the client.
What are the models of the router/client, what speeds/standards do they support ?
What speeds are you getting when close to the router, vs. when at the destination ?