Can you link me to the Three contract you are using?
It is this one LINK I have started with a 1 month contract at £26 if it proves successful i will switch to 12 or 24 month to bring the cost down a little, or maybe even try Smarty which looks like the same thing for £18.75
The router is network unlocked so I can try different networks, only pain will be moving the external antenna, but can do initial tests on a mobile phone to test signal etc.