Sorry for the late reply.. The tariff was originally only £15 a month til they upped it to £20. The tariff Is called "All you can eat data" or "All you can eat 20" I used wifi as my connection. Have used several handsets. Galaxy S3 mini, Huawei y300 y330 y500, and now a nokia Lumia 930. I did not use any 3 mobile app. Various OS's used doing this. From ice cream sandwich, jellybean, all way up to 5.1, and win 8,8.1 and win ten.
You can also change the "APN" settings to bypass the networks filter.. I think the default is 3.co.uk, edit it to "three.co.uk" (or vice versa) and no more "sorry over 18 only" (Don't mean for "adult" stuff, more for streaming sites etc)
There are NO pay monthly/contracts/sim only that offer unlimited data. Giffgaff offer it but not for tethering.
As for being detected by networks, its all complicated stuff, but I figured tethering from an android device to an android device, or windows to windows was what kept me under the radar. Plus the fact I never EVER downloaded big files.. PLENTY of streaming though.. decent speeds too. Three (for me) are the fastest..
I couldn't offer an explanation as to why some say it doesn't work for them? Are these people using CRAPple devices? I know they removed tethering settings til a few years ago.
For those that cant do this successfully, maybe think about using a google chromecast to "cast" (project) your screen onto a bigger device. I have an old cheapy Huawei android phone, with a chromecast I can put the whole screen onto my 51" plasma within seconds, whatever's on the phones screen, is displayed perfectly on my TV. Chromecast sticks cost around 20/25 quid and usually include an offer for 2 months free NOWtv..
Finally, if you do hammer the sim card and do receive warnings, just bin the sim and pay a pound for a new one every 4 weeks. You could just use an old phone purely for hot spotting.
The people who say they cannot do this, can you post up what device/os/details etc please, really puzzled as to why you cant enjoy unlimited data?!?!??
Happy new year all!