Their Tech?

I agree, plot hole #7 out of 30.

Where are they getting their tech (nanites, stasis field, etc.)? Don't be surprised if they never plan to answer that question.

I think this is two separate questions.

One answer might be that early travelers set up secret factories, using contemporary technology, that, in a process of bootstrapping, built the tools that can build the gizmos they know how to build, from the future -- like the stasis field we saw in episode 1.10.

However that leave a more difficult question.

The giant space rock that practically wiped out civilization arrived in 2017 or 2018? I suggest that if it was that devastating, it would have put all blue-sky scientific research on a centuries long hiatus. So, even if they are coming from hundreds of years in the future their technology shouldn't be that far beyond the 2017 level.

This was pretty much answered in Episode 6 ("Helios 685"). They've been sending people through for a time that have been able to assemble future technology in the present.

Worth noting is that some major technological advancements that we're working on is either a matter of miniaturization or limited by our current understanding of physics. In fact: Some technological marvels theorized in science-fiction may never come to pass because we're governed by laws of nature.

That said: If they, in the future, make these groundbreaking discoveries they too are grounded by the same laws of nature. For instance chemical elements that allow for advanced nano-technology would probably exist today as well. It's just that we wouldn't have figured out how to utilize them properly.