I stay in South Africa, so am using .co.za links. you should find something similar in your country
You want it portable, which limits the amount of oomph you can put out via the antenna, which limits the coverage distance. Most portable sim card based mifi devices firstly have got a limited range, and then secondly can only handle a limited amount of connections. I assume (correct me if I am wrong) that you want it mobile for "allways on" connection as you are traveling, or possibly to provide connectivity when you are "at rest"
That said, instead of using a single solution, I would suggest looking at a split solution = one part for the 3g/4g sim connection (basically you are only going to need a modem converting to a network point), and one that will be the dedicated access point. for power, many devices doesn't have to be on the mains - just ensure you charge a battery/powerbank, output at the correct voltage, and use that as the source.
... and then you connect both of them with a network cable to one another.
(you also get simple network hubs running of usb), a friend of mine can run his 5port dlink hub/switch from a usb - meaning it will work from a powerbank as well - this might be a great add-on if you want to extend your travel network later with an extra Access point
For a wireless access point - something like this https://scoop.co.za/ubiquiti-unifi-e...-range-ap.html
important specs - can handle 100+ concurrent connections, 300Mb/s - so still faster than your typical 3g/4g, and long range - Range: Approx. 600 ft
more important - it uses 24v, but this is a POE device, so you either provide it with 24v through the correct connector, or you inject power over a network cable
the next component is something that can either connect via 3g/4g or take a 3g/4g connection, and provide an output to a network point - RJ45.
example - this one will use a 3g modem providing a 3g/4g connection, but I am quite sure you will get something that is integrated. Other Computers & Networking - M1 Portable 3G WiFi Hotspot IEEE802.11b/g/n 150Mbps RJ45 USB Router for sale in China (ID:238062777)
the advantage of splitting the 2, is that you can utilize 2 components, that work very well for the specific task they do, instead of using a single device that mediocrely combines both components
also, if you do arrive anywhere where you have another source of broadband, you can possibly connect the one component (wireless access point) only.