Can not connect to personal hotspot on 6s

Since I renewed my contract to sim only with data last week I can’t connect to my personal hotspot on my iPhone 6s. My other devices try to connect but comes up with failed to find IP address. I have been to a O2 shop and they said it’s either iOS at fault or my phone and tried to sell me a new phone. I have all the new iOS updates and I have reset the phones carrier settings. Any help would be appreciated.