I can’t believe your ISP was unable to resolve this issue for you, that is very shame full on their part.
I think you know this but I’ll say it anyway just to be sure we understand one another
You are connected through a modem and then you have a router connected to that? When you are just connected direct to the modem without any router then the commands you have tried should change that IP Address but that isn’t always the case, some dynamic IP Address are assigned to that user for a number of days so that would make them command useless if this is the case with you
If you are connected through the router then you would be connected to your LAN and all the commands you have tried would only change your LAN IP Address and not the public IP Address. Have you tried to turn the power to the modem off and then turn the power off to the router, wait a few minutes and then re power up the modem and connect direct to that? If you haven’t then please do try it
Let me know how you get on, best of luck.