Downgrade iOS 12.4.8 to iOS 11.3.1 – iPhone 6 (Without SHSH blobs)


Is there any way to downgrade iOS 12.4.8 to iOS 11.3.1 on iPhone 6 (Without SHSH blobs). The firmware is not signed by the apple anymore.

I’m stuck at iOS 12.4.8 on iPhone 6 and cannot use Wi-fi Calling, was able to use the feature back in 11.3.1.So need to go back. Please help.