No outlandish overage fees, data rate beyond the 5GB plan is same as for the plan.
I use ATT data services quite a bit in isolated areas where other carriers don't have a presence.
Don't expect streaming video or good VOIP performance, because if the location is so remote that Verizon and Sprint are not even there, it will not have 4G, may not have 3G and Edge is not fast enough for streaming. If it doesn't have Edge (which is 3G by global standards, called 2.5 here although that has no meaning as a standards definition) you will be down to GPRS. That will get you to the Internet, but most web pages today have enough MM content that they will time out trying to load pics or videos, at GPRS speeds and bandwidth.
If you really need Internet, look into a mobile satellite provider. You might have bandwidth issues there, too, if you want to stream video.