Cannot connect to wifi via cellphone

[Sharlene1981]

So we finally could get fibre and I was able to connect to the wifi no issues but this whole day when I want to connect it says denied access to network. I have forgotten it and tried again will stay connected for may 2 seconds then I get that denied access message again. Have restarted phone and even reset my network settings but still same issue. All my other devices have no issues but only my phone. I still have my other router from Telkom as that ends the end of the month and can connect to that router no problems. My isp says it is my device but it cannot be as like I said I could connect to my Telkom router fine. The isp also said that I should try and change my ISSD name as that might solve the problem. Has anyone else had this problem. If so some help would be appreciated.