This use to be a common problem with some iPads and routers. I haven't heard of it lately, but this is what used to fix it, at least for a time.
1) Go to settings on the iPad and forget the network. Do this by going to wifi settings, tapping the info icon (I) next to your connection, and choose forget network.
2) Power down the router. Wait a minute or so and power it back up. You can also power down and up the iPad while you are at it, just as an extra thing that might help.
3) When the router and iPad are both back up, rejoin the network.
Routers can retain a memory, of sorts, about the devices that connect to them. Bad or conflicting information can be stored, and cause connection issues. The above steps helps the router see the ipad as a new device, and assign it an IP address clean of whatever issues the router had before.
It also reboots all the things, which is a pretty good first step when encountering many problems.