Excuse me, this dish is cold.
FRED-do — two syllables, double 'd', stress on first syllable.
When a hot dish arrives at the table clearly cold — possibly because it sat too long in the kitchen or service was slow.
'Freddo' means cold. This is a factual statement rather than an accusation — start with 'scusi' to keep the tone polite. The restaurant will almost always offer to reheat the dish or replace it without question.
Il piatto non è caldo — è possibile riscaldarlo?
The dish is not hot — is it possible to reheat it?
More explicit — requests the specific action you want taken
La zuppa si è raffreddata.
The soup has gone cold.
Describes the situation without directly asking for anything — leaves the initiative to the waiter
Può portarlo in cucina a scaldare?
Can you take it to the kitchen to heat up?
Very direct request — appropriate after a long wait
Hot food served cold is taken seriously in Italy where the quality of the eating experience is paramount. Any decent restaurant will immediately offer to fix it. If they do not, it is a sign that the establishment does not meet Italian standards of hospitality.