Just reading this thread and may I add my Three pennies worth!
I have the older Huawei E5573 mifi with just a couple of LEDs but it works on the 4G network when available but not abroad. It helps to have the Huawei app to keep track of your setup and usage
I have bought two 12gb/12 month data Sims from Mymemory web shop at a considerable discount around £24.99 to 28.99. The Three add-on packages are a rip off and only last 30 days so if in the UK just buy another sim card.
Recently touring Ireland N and S I took an EE data card which costs £1 then you top up as required. It was £19 for 10 GB for 30 days or the top up I bought was £22 for 10gb for 90 days and I was given and extra 1gb with each top up.
It has worked well and I can easily switch SIM cards with no setting up APNs needed. The EE app works fine for monitoring the data usage.
Originally I had a Three monthly data contract £16.01 for 10gb but when not touring I didn't use any data some months . My son borrowed it for a holiday abroad and went over the monthly limited by a few Gb and it cost £36!
Three's add-ons are very expensive and only last 30 days. I have a Three Payg SIM in my phone and they want you to buy an add-on for feel at home use abroad but we have rarely needed to use our mobiles when abroad so just pay the normal roaming charges which aren't too bad while we are still in the EU!