This was what caused a lot of issues with my dad. He was jumping in taxis, going to the Drs then going to the bank instead, or getting straight out again... that was after he drove himself and lost his car at the Drs where it was duly towed.
He had no organisation over appointments and my local family had no patience to take him. I was due to get carers in to go to appointments with him and take notes, but we ended up in crisis mode when the care company went to assess him and he hadn't eaten all day, having had his food cut off by the independent living centre (they had a canteen where he was taking all his meals having lost food preparation as a concept some time ago, or perhaps never fully picking it up to be honest).
I was getting calls from podiatry or Drs or whatever, your father has not shown up, we're striking him off services etc.
Now, thank goodness, the home send someone next door where the Drs is with him and they keep a calendar for him too.
You may have better luck sending someone and also get your Dr to put your mobile number as text reminders and send you the appointment letters. they are little good going to PWD.
To be honest all these letters used to stress dad out so it's load of all our minds.