Help removing edgun lelya regulator from plenum!

Have brought a huma regulator and have problems trying to remove the original regulator (red box below) from the plenum. I know it should just twist off from the online huma video but I think one of the previous owners must have screwed it on real tight or used loctite - any tips would be appreciated to remove the regulator without destroying it! Difficult to get it in a vice as its smooth and circular.

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