Is my fasts not accepted because I don't wear hijab? (I wear very modest clothes)
What exactly is the punishments for not wearing hijab?
Covering the 'awrah is not a requirement for fasting (unlike prayer). So your fast will be valid. However not wearring hijab in front of non-mahrams is a sin. And continuing this in Ramadan means that you may be deprived from the reward of fasting as you have not refrained from sins.
من لم يدع قول الزور والعمل به فليس لله حاجة في أن يدع طعامه وشرابه
Whoever does not give up forged speech and evil actions, Allah is not in need of his leaving his food and drink.
رب صائم ليس له من صيامه إلا الجوع. ورب قائم ليس له من قيامه إلا السهر
There are people who fast and get nothing from their fast except hunger, and there are those who pray and get nothing from their prayer but a sleepless night.
The punishment (in the afterlife) for not wearing hijab is Allah's choice. It is known that in general any sin may be punished in Hell fire unless Allah forgives it, and abstaining from Hijab is included in it as it is disobedience to Allah.
For example it comes under the warning of the following general verses:
ومن يعص الله ورسوله ويتعد حدوده يدخله نارا خالدا فيها وله عذاب مهين
And whosoever disobeys Allâh and His Messenger and transgresses His limits, He will cast him into the Fire, to abide therein; and he shall have a disgraceful torment.
وإن الفجار لفي جحيم
And indeed, the wicked will be in Hellfire.
And more specifically, it also comes under the threat of the following hadith:
صنفان من أهل النار ... ونساء كاسيات عاريات
(among) the denizens of Hell (are) ... the women who would be naked in spite of their being dressed
وقيل معناه تستر بعض بدنها وتكشف بعضه إظهارا بحالها ونحوه
And it is said that it means covering part of the body and uncovering part of it