How do I reset my phone to the towers ?

[shultj2]

I know there is a number to dial to reset you phone to the towers when you are in a new area Can someone tell me that number? I have used it before but it is a while ago

My phone is only getting 3G at this time and I know 4G and even 5G are available  Not to mention up till about a month ago I got 4G   and My husband gets 4G on his phone exact same phone bought at the exact same time