So what should you say?
If you just say “Je ne comprends pas” (I don’t understand), the person will think you didn’t get anything, and is very likely to just switch into English, or give up on you.
So, instead, say, “Je ne comprends pas bien” (I don’t understand everything), causing your interlocutor to reformulate his sentence, using other words. And do not hesitate to say, “S’il vous plaît, parlez plus lentement et plus fort” (Please, speak slower and louder). Speed and bad hearing are often a big part of not understanding.