How to keep a LDR during Covid-19?

That's probably the hardest thing to maintain.
Keep some daily text things, like good morning/good night kind of things.

Don't call everyday, unless you have a really awesome life and tons of shit to tell, because calling without having anything to say will make it painful and boring.
If you are good at crafting things, craft some gifts and send them to her (if packet deliveries are a thing rn). Or find anything else you could send her that would means she has your attention.

But the main thing is : both need to try to maintain it.