ADHD is much more than brain fog, so I am not convinced that POTS causes ADHD. Surely POTS can cause some symptoms that overlap with ADHD, but not all of them.
I have had ADHD symptoms back as far as toddlerhood. I was diagnosed in my teens, but nobody thought to try medication until much later. I didn't have any symptoms consistent with POTS until about 3 years after I started taking Strattera, a non-stimulant ADHD medication.
I recently switched from Strattera to Adderall for ADHD. Interestingly, I am feeling 100% better POTS-wise. I do not know if it is because I am no longer taking a norepinepherine reuptake inhibitor (NE was leaking out of the synapses into my blood perhaps?), or if the Adderall is helping my poor deficient veins to constrict properly. Both are possible. Adderall has fewer side effects than Strattera, but I liked the 24 hour activity of the Strattera. I'm learning to deal with it, though.