Cowshed
The answer depends more on your narrative and personal choices than a “English language rule for info dumps.” I agree with the post above that you should try to avoid info dumps. If you must use one then ask yourself if the description is happening in the past or present. Use the appropriate tense.
As for switching tenses, in general don’t. There are certain cases where you can (someone mentioned a flash beach earlier, which is a case where you can use past tense in a present tense story), but you want to mostly stick with one tense to avoid tense switching that confuses the reader.
There is no rule that dialogue is present tense by the way. It’s just very rare that someone talks in past tense in stories. Here are some examples to help you select the correct tense:
Description taking place in the past:
A long time ago, there WAS a war that DEVASTATED
the land.
Vs Description taking place in the present:
Right now, war IS DEVASTATING the land.
Now I’ll try to cover speech. Speech is tough because even if the speech happened in the past, the speech can be present tense because the speaker is talking in the present tense. You can have a speaker talking in the past tense though! It’s not ALWAYS present tense as a “rule of the English language.”
Present Speech in present tense:
“This war IS DEVASTATING the land,” he SAYS.
Past speech in present tense
“This war IS DEVASTATING the land,” he SAID.
Note that the words said didn’t change. The only thing that changed is when the words were said. Now I’m going to change that.
Present speech in past tense:
“A long time ago, there WAS a war that DEVASTATED the land,” he SAYS.
Past speech in past tense:
“A long time ago, there WAS a war that DEVASTATED the land,” he SAID.
Now we flipped the tense of the speech because the person talking is talking about the past.
Tense is all about when the subject matter happened. Anything happening now gets present tense. Anything that already happened gets past tense.
So what tense is your info dump? Whatever tense the rest of your story is going to be, probably. Unless you are starting with a flashback to give an info dump or something that makes the info dump happen at a different time than the rest of your story.