Isn't does the negative imperfect past? I assume it is the same as the doedd I see on verb tables online.
You assume incorrectly.
I'm not sure what table you look at, but https://en.wiktionary.org/wiki/bod#Conjugation , for example, has "does dim" listed in the present tense in the "colloquial forms" table (you may have to click on the "conjugation" bar to open it up) and "doedd e/o/hi ddim" for the past.
Ah, I had not noticed the colloquial forms table. The question, of course, is whether they are colloquial for this region or that region, for this village or that village. Clearly, this Duolingo team is using a particular standard, and it would just be nice if they would install some sort of Tips and Notes to indicate what that standard might be.
does dim... is common all over Wales in the colloquial register, with just the pronunciation varying a little.
can this mean "it has no signal" when talking about your phone, or only about someone else who can't get a signal on their phone?