Three Router

Three is the parent company of Smarty, so a three router should work. 

Other routers would need to be unlocked for use with any network. 

Bear in mind that section 5.15 of Smarty’s terms and conditions state that Smarty Mobile is not to be used as a permanent replacement for home broadband. If you are suspected of doing so, Smarty reserve the right to suspend your service. 

Responsible use – Unlimited Data Only Plans
5.15 Our Unlimited Data Only SIM plans are for personal, non-commercial use only and can be used in SIM enabled tablets or
laptops and portable mobile WI-FI devices and is uncapped for this use. This SIM card should not be used as a permanent
long-term alternative to home broadband. If we determine in our sole opinion that your use of the SMARTY Services is in
breach of these Terms and Conditions and/or exceeds that reasonably expected of someone using the Service for legitimate
personal, non-commercial use, then we may at our discretion terminate (in accordance with Section 9), or Suspend (in
accordance with Section 8), or restrict your SMARTY Service. As an example, without limitation, we consider that if you
regularly tether more than 8 devices that this is unlikely to be legitimate personal, non-commercial uwe.

If the mobile broadband router is not network locked Ade_Smarty_User, it should work with a SIM from any UK service provider.

A word of warning though - some providers, like SMARTY, have terms & conditions stating plans with unlimited data allowances should not be used as a long term replacement for landline broadband.