No internet on hotspot despite correct settings

Move this week to Smarty from Three on a full unlimited data package. Phone transfer went ok for both me and my wife doing the same. However, neither of us can get any data connection through our hotspots between iPhone & Mac or iPad. The online help gives the usual useless advice and my mobile data APN settings were automatically applied correctly (per this https://help.smarty.co.uk/en/articles/1155220-using-the-internet-after-you-ve-joined-smarty) during the transfer process. How can I fix this before I have to change providers again ?