The Constants of Nature are "fixed" into physical Laws within the Universe. The fixing paramiter is of course Time, we place a value on testable quantities "now", in the present time.
In GR and within QM, both allow the Constants of Nature to be fixed, but both QM and GR allow the "effects" of the constants to be varied!
In QM, there is the Law of observation, in GR there is also the Law of observation, both theories are WRT position and momenum, encased within a reference frame of Time, present-time.
The constants of Nature are Observationally varied by the Observers who are detecting them.