Today we discovered that a drain in our fibreglass shower stall is loose, moves freely around the black sewer pipe and up and down approx. 1/4". Shower and drain are approx. 15 years old and we don't know models of types of either of them. The drain is ABS ("use ABS contact cement only") white, looks like 1 piece with steel snap-on grate and black gasket between top drain flange and shower base. No silicone caulking or putty can be seen anywhere. Gasket does not seal anything because is loose.
We would like to remove the drain and reinstall tightly or to replace it with the new one. But first how to remove it without damaging shower base? Tried to pull, turn, nothing works.
I would appreciate any help.

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