
DAP has never done anything for any of mine. Pet remedy diffusers have a much higher success rate from what I've read and heard.
There are plenty of supplements/remedies to help though, as well as management techniques - it's about finding what works for your dog. It's too late to start proper desensitisation/counterconditioning work now with November just around the corner, you'd need to wait until New Year is passed now but you can get other things in place.
So, management techniques:
Making a bolthole - a crate covered with heavy, dark blankets; or blankets over two chairs, or moving a side table away from a sofa/chair arm and draping a blanket over that (be creative!). Or, a quiet area such as the bathroom can be good - darker than other rooms and some dogs prefer to just take themselves away. Ditto under the stairs or similar.
Not ignoring the dog - you can't reinforce fear, so don't be afraid to reassure your dog if they need it. I find mine are much better if I stroke them gently and talk softly (although I do still avoid saying 'good girl/boy' or similar just in case).
Distraction - if the dog will respond, do tricks or basic training to keep them occupied.
And if needed, supplements (by no means an exhaustive list):
DAP diffuser/collar/spray (if it works)
Pet remedy diffuser (ditto)
Skullcap & valerian tablets
Valerian tincture (works faster, I find it much more effective)
Serene-ums
Calmex
Zylkene
Melatonin (my personal favourite - does not sedate, just takes the anxiety away so the dog doesn't care about the trigger and works within half an hour - 80% success rate in dogs)
Then we have the strong stuff:
Valium (diazepam) - I'm not keen, can cause dogs to get wobbly and hallucinate (although that isn't that common I don't think) but does help a lot of dogs.
Xanax (alprazolam) - much better choice for noise phobias, effective for that particular issue with much less sedation/wobbliness. I use this for Phoebe on top of the melatonin, although with that I rarely need to use it these days.
It's all horses for courses, of course - it took me 3 or 4 years to get anywhere with Remy from the same level of distress you describe, and it was S&V tablets that finally started the process. Valerian tincture really made a big difference for him, and these days I use melatonin as it works the best for him (likewise Phoebe, and my friends' dogs). Zylkene is a good one to try but I'd be starting it within the next couple of weeks to give it time to take effects; everything else on that list is same-day effective, some faster than others.
Re. prescription (valium/xanax) - I avoid them if possible. Phoebe is an extreme case with her noise phobias, they are many and varied and severe so she does have it in the cupboard but it's rare she has it now. Strictly one-off use - these can be used for a couple of weeks but then the dog MUST be weaned off them as they can be addictive and prolonged use can cause an increase in sensitivity to triggers. If I use a small dose for Phoebe more than twice a week she becomes more reactive.
With any supplement, always try it BEFORE any fireworks are happening - ideally in the daytime, when the dog is happy and calm. This is to check for any adverse effects when the dog is not already struggling to cope with something else and infuriatingly, it is something that many vets do not suggest :-(