Sim is not Supported in unlocked iPhone

Hi Guys, recently I got iOS 13 update on my iPhone 7 Plus. After updating my device feels glitchy so I decided to factory reset it. After resetting when I put back my Sim Card in then it shows an error saying:

"The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information."

I had been using this sim in this phone for 2 years and I also factory reset my device many times before but this time it showing me this error. I searched online and everyone says it means my device is locked. If it was locked then how I was using it all those times and I also factory reset it many times before. What changes this time?

Help would really be appreciated.