I have asked my carrier to unlock my iPhone 7 Plus so I can use a different SIM card and they will not unlock my phone, what can I do to unlock my iPhone

Only the carrier can remove a carrier lock on a device. Many will do this after you have satisfied the conditions of a contract, though some may not do it at all.

How to unlock your iPhone for use with a different carrier

There are several tech companies that sell programs or SIM cards that will allow you to unlock (carrier) your phone. You’ll have to do the legwork yourself to find one.

i purchased a program to restore an iPhone 5s that froze in recovery mode during an iOS update. I replaced it with a newer iPhone and set it aside for a little over a year, I decided to give it to my granddaughter if it could get it restored. I spent countless hours here in Apple Support and online looking for a remedy. I found a program to do it, purchased it and it restored my 5s.... the catch here, it was an AT&T proprietary phone with the AT&T logo and tone on startup. Long story short, the program wiped all proprietary data off the phone.

I thought no big deal, powered it up using the SIM card I had in my other phone and the phone read no service on the screen. It was around 2AM. I went to the local AT&T store requesting a new SIM Card, they told me my service had been suspended for using an unauthorized phone, even mildly suggesting the phone was stolen. After a few hours of conversation and proof that it was indeed my phone. My service was restored and I was on my way. So... the moral of the story, unless your account and phone are unencumbered, you May end up with a bigger problem.

Again, Bear in mind I don’t advocate usurping your carrier if you owe them money on your account or for your phone, though I am aware of at least one carrier that refuses to unlock proprietary phones.

And it’s not AT&T.. Even though the phone was unlocked already, I asked them to unlock it, took two days and they informed me it was clear.