My 5 year old is having similar toilet problems. He had improved so much, the contact with dad restarted and he's gone backwards to several accidents a day. I've had him checked for a water infection to rule it out and been working with the toileting nurses linked with the school/local area. As some of issues prior to this were linked with constipation. Now I think it is solely to do with his emotional needs.
From what I've found out it can be because they are distracted and therefore don't think about the toilet (distracted by big feelings, new places, seeing daddy is exciting, etc, or in both our cases it seems tiredness is playing a part). So set one hour alarms on your phone and gently remind.
It can be a control thing, that life has changed so much and they aren't in charge of anything. So give them responsibility to be in charge as much as possible. Getting dressed for example, or giving them two things to choose from..
It can be not drinking enough because it isn't a focus for them with everything that is going on. So encouraging, new water bottle, same at other house.
Praise is important, praise the smallest improvement. Pebbles in a jar, stickers in a chart and reward. Reward for going, reward for drinking, reward for telling you they need to go or had an accident (my son has now started hiding it and just getting himself changed).
Don't make it a bad/thing to be ashamed of.
I have to model what I expect him to do. So when my phone alarm goes off I go to the toilet too. I got a new water bottle and we both challenge each other to drink.
Also with all of this certain things can then irritate, so no caffeine, fizzy or blackcurrant.
I think the big thing I've put it down to is worry and anxiety. There's been a lot of change, my son on the surface has coped well but he hasn't really, he's just not old enough to express himself. So it's all built up inside and come out in a physical way. So giving control and the time and space to open up has really helped. I've written a list with little pictures so he knows the order of how to get ready in the morning, I no longer nag, he wants to be in control and do it, it's helped us spend our energy on talking and connecting. He misses me when he goes to his dad's so we read books about two homes, we draw and talk about our feelings, when I feel the same way I tell him so he sees it is normal.
It is so hard. Feel for you as I am really going through it at the moment and the art back has been hard. But reach out for support with school if she has started or is about it, or the GP. They might be able to offer guidance. Wishing you luck..