How long has s/he been in the care home? Assuming s/he is there already. Speaking from experience (a parent and an in-law) it can often take a while for people to settle.
I’m a bit uncertain as to why the person needs a secure care home, if it really is early dementia. Some ordinary (non secure) residential homes will take people with early dementia, if it’s early enough that they’re not going to bother non-dementia residents. We had an aunt like this - however the care home did make it clear that we might need to move her later, and we did, but she was there for about 2 years.
As for assisted living, presumably you mean a flat with assistance laid on - frankly I’d be a bit dubious. For a start, a change of surroundings can be very difficult for anyone with dementia to cope with - everything in different places, different cooker, etc. A failing memory can find it very hard to manage outside very familiar surroundings.
Secondly, dementia can get worse quite quickly, and what
I’ve heard of more than once is the resident causing problems by e.g. repeatedly knocking on other people’s doors because e.g. they’ve locked themselves out, often at unsocial hours, or wandering off and getting lost, etc. And ultimately being asked to leave, because they’re bothering the other elderly residents.
Whereas in a good care home, and despite the horror stories there are some very good ones out there, there will always be someone there to help and reassure.
I do recommend the Alzheimer’s Society Talking Point forum, for carers of people with dementia. Whatever you’re going through, someone else will have been there. I can’t say that there are often easy answers, though.