Wifi repeater

OK. You can do this from two different directions. You can boost connection to the CG and connect your phone to wifi, where the router is primary internet connection or, if CG is bad or non existent, you can connect your router to your 3g/4g phone hotspot, where your phone is primary internet connection.

You can set it up for $100-200, depending on what devices you pick.

Read/Watch HERE.

Have you looked at the Ubiquti Nano Station (directional), or Pico Station if you prefer more power with a small Omni-directional antenna, or the Bullet if you want it to be more like the ones you posted? They are much cheaper (under $100)and not as hard as they look to set up. There is a good you tube video of a fulltimer setting one up to do what you are wanting. They are just as powerful.
I just set one up in my RV and it brings the signal to my regular router for me to use everything inside my RV.

We have the Rogue Wave and have had very good results with it.

Just remember that although these devices will bring in a signal that is too far away to get otherwise, they are not magic. If the campground has inadequate bandwidth to their internet provider, these won't fix that problem.

If you can get good download speeds at their office, but not at your campsite . . . these will help considerably.

Also look at Hawking Electronics. I have their combination wifi amp/repeater. Works great.