How to plug my computer into any power source and bring my own internet?

For a couple of months, I'll be in temporary places. I'll have my Gateway mini tower with me.

How can I plug my computer into any power source and bring my own internet?

I'm thinking there is a device I can plug in and sign-in via magical sky signals. Will you give a brief rundown of how people may travel with their computer like I do without relying on in-house internet or buying a laptop or phone?

Thanks for the advice!
Added (1). I should be more clear. I can't hang out in a coffee shop or bookstore with my desktop computer. :)

I need to plug in at, say, a hotel room or office for a whole day.
Added (2). A mobile hotspot, aka MiFi device, is what I am looking for. Not that I'm going to buy from AT&T, but this is what it is as an example device (in case anyone else is looking for the same thing:
https://www.att.com/shop/wireless/devices/novatelwireless/mobile-hotspot-mifi-liberate-graphite.html

Also, an excellent options guide here:
http://www.att.com/att/planner/

Again… I'm not vouching for AT&T, but their information was easiest to find and understand (esp that planner).

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