Need Help w/ Long Range Wireless Network (300ft-500ft)

[fire7882]

Hope this is the right section.

I'm looking for equipment that would allow me to send a wireless signal about 400 feet to another building on my parent's property. They currently have AT&T 6.0 DSL and their modem acts as a router. I would assume I would need an Access Point at the modem/router location and then something else at the computer location that is 400 feet away. Is there another standalone device I could use to receive the signal and connect the computer to that with an eithernet cord? Any specific product recomendations?
Also, there are 2 other buildings/cabins about 1000 feet from the modem/router. Would it be possible to have a signal reach these locations affordably? I provided an image below for reference. The 2 blue circles are the primary locations while the 2 white circles denote the locations where it would be nice to have internet. The router/modem and access point must be located at the top blue circle.

Thanks in advance for the help