We lived in Dubai and this was one of the main reasons we left.
Quality SEN support is hard to access. As you have found there aren't really separate SEN schools - I heard of some only for locals.
Despite many international/british schools saying how SEN friendly they are in their glossy marketing the best schools are oversubscribed so will have a cap on how many SEN students they admit. There are usually admissions tests/interviews even for non-selective schools to assess your DC .
Everything costs a huge amount over and above school fees. Parents usually have to employ the 1:1 LSA directly - typically a minimum of 5000aed pcm, rising rapidly for anyone half decent) and the quality of candidates is very varied - finding one with qualifications is very difficult - we were lucky to find a lovely lady with excellent experience for DD but she wasn't a specialist. Some schools will arrange this on your behalf and only employ ladies who already have a visa (ie their husbands) in some cases you need to be their visa sponsor and pay those associated costs (and have responsibility for them if they do anything illegal).
It's rare to find a native English speaker in these roles. All forms of additional therapy are extremely costly, and for ASD/ADHD there is a strong focus on ABA therapy which would be controversial here.
I think there can be issues getting your usual medication for ADHD but this wasn't applicable for us. Anything related to a psychological disorder/mental health (as many SEN diagnoses are classified) is also usually an exclusion from medical insurance - check what's offered in your package with a fine tooth comb.
Children with SEN are referred to as 'children of determination' so you may find more info searching that term.
There is a private Facebook group called 'CANDID- family support' which used to be a good way to connect with other SEN mums - also search on British mums Dubai group which is massive.
I am sorry this sounds very negative but I wish I had known it all before going. You need to weigh it up very carefully. You haven't mentioned what salary you or DH have been offered but that is an important factor. If one or both of you have a great package and high salary (eg 100k aed per month) then many of the issues can be solved. If you are a teacher or similar on 15k you won't be able to pay for what your DC needs and it will impact on your ability to make the most of the experience.