It is highly likely you are referring to one of the following two models, which have similar naming conventions and are commonly searched for factory reset codes:
*#0*# or ##0000#
*#*#3646633#*#*Connectivity -> CDS Information12345678If the screen is frozen or the menu is inaccessible, you can perform a hard reset by cutting power to the unit. grundig gx3800 factory reset code verified