The spare appliance must provide appropriate conditions for the actual load. Squeezing every item into it can block airflow or prevent closure.
What the observation tells you
It also cannot preserve frozen storage simply because its interior feels cold. Frozen food moved there becomes a thawing task with different timing. An old spare appliance may need startup and temperature checks before it can be relied on.
A useful next step
Separate the chilled and frozen storage needs, confirm the spare unit's suitability and follow food guidance. Do not overload it in a way that creates a second cooling problem.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.