iPhone 7 Plus locked after inserting a SIM card

If you find Settings > General > About > Carrier Lock (or Network Provider Lock) > "No SIM restrictions" is good. Anything else shown is Not Good.

If the iPhone is carrier-locked and you can’t get a refund for your carrier-locked iPhone purchase (ro the seller gets it unlocked for you), then you got scammed. Most carriers won’t deal with some third-party looking for an unlock.

How to unlock your iPhone for use with a different carrier - Apple Support

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

You may have other issues as well. The iphone 7+ is an antiquated phone and can't run an operating system newer than iOS 15.8.3. Even if you can get the SIM to work, many apps won't run on a version that old.

Take it back and getting your money back.

“Hello, I have a problem. I purchased a used iPhone 7 Plus without a SIM card. When I inserted a SIM card, it showed me that the device was locked to the company name 50.0 Carrien, which is the company's supposed number. I'm very confused and have contacted several telecom companies trying to unlock it, to no avail. I hope you can help me, and this is my hope from your esteemed company.”

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Getting Into a Locked iPhone:

Where did you purchase this iPhone? This was likely locked using Find My. If they do not sign out of iCloud on it, then you will never be able to own this device. This is the site the company should have used prior to ridding of it: What to do Before you Sell, Give Away, or Ttrade in your iPhone or iPad - Apple Support. They should have known better.

“Thank you for your prompt response. All my respect.”

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You are welcome.

Note: It’s quite obsolete. So, if bugs were to come about, then bug fixes would no be available. Apple typically provides updates for 7 years, and then they stop doing so.