Hello, Captain, and welcome to TP.
I wanted to echo the previous suggestions of either arranging a home visit, if that's possible in your situation, or writing to the GP with an account of behaviour/issues/concerns. Others here have reported that their family member was amenable to following "doctor's orders" (and indeed, my own mother, with dementia, would mostly do what the doctor said).
One of the reasons this sometimes works for some people (note disclaimers!) is that when the person is resistant to doing what family members ask, putting the burden on a third party or outside person is often helpful. In a workshop I went to they phrased it as, consider not only the message that has to be delivered, but also the messenger and then further stated that with a parent, often the adult child is NOT the right messenger.
Also for many people, "the doctor" retains some clout as a perceived authority figure. If I asked my mother to go for a test or appointment, for example, she would refuse and get upset, but if "the doctor" told her, she would comply. Whatever worked. So I would have conversations with her GP before and after appointments, and we would discuss what needed to be done, or he'd let me know what he would be discussing with her in the next appointment (this was with her permission).
We also used the doctor as the messenger. For example, me telling her she couldn't drive any longer was not going to be good, so the doctor was the one to deliver that news (his idea, bless him). Every time she would bring up how she still wanted to drive and would try to blame me, I would say, I know you like to drive to the shops and feel you can still drive safely. However, the doctor says no driving for right now. Well, when can I drive again? When the doctor says it's okay.
So that was probably more than you needed to know about my mother; sorry! I just wanted to give an example of how a helpful GP can make some things easier, for instance, if your mum needs some testing done or an assessment or a memory clinic appointment or whatever. You don't have to be the one to say that has to happen! Or if she asks you why she has to have whatever appointment, you can say, I don't know, Mum, but the doctor said we need to do this. A lot of people with dementia claim there is nothing wrong with them, that they're fine (when they are clearly not) and that can make it difficult to talk about things like doctors appointments, because for them, there isn't any reason to go. My mother would get agitated about appointments unless they were framed as "a routine check" or "the doctor has to see me every so often or he can't prescribe my medicine" or something similar.
Best of luck to you, and if you get a chance, please let us know how you get on. There's a lot of good information and support available here on TP.