There is usually two sides - one that argues that it doesn't exist, one that argues that it does indeed exist.
While I usually keep myself out of that kind of conversation, I think I found two rather... simple and objective explanations for all of it. The two are not seperate theories, but they work in conjuction with each other.
If you are considerably better than your current ELO there is no ELO Hell. I am around high plat or low diamond in skill and I got placed in Silver 1. In a few days (under a week) I have 28 wins and 14 losses - I'm currently at Gold 1. This is unsurprising, because it would take immensely stacked teams again and again to keep you from advancing, which is just statistically unlikely.
1. If you are only a bit better than your current ELO, then your potential to carry the game is barely above average. If we assume every single player in the world has the exact same luck with teammates, then you will (slowly) rise, nonetheless. However, this is not the case. Since you are only slightly better, even a tiny fluctuation in your luck can completely block you from advancing. Because the 1 out of 20 games you actually carry because you are that tiny bit better, is lost due to your bad luck again.
2. To understand this effect fully, you must realize that League is played by millions of players. What this means is best shown by an example.
Imagine that there is a 10% chance you lose a game because of having an AFK player, one with internet issues or whatever other reason.
The chance of getting such a player in THREE GAMES IN A ROW is merely 0,1% - however that also means that 1 out of 1.000 players has this bad luck. Given that there are 30 million League Players this means that 30.000 Players have this bad luck. 3.000 even have such a player FOUR times in a row, and so on.
The ones coming to the forum usually actually have luck as bad as they say they have. Because it's simply statistically logical. With so many players out there, there must be ones with very bad luck. And they are the ones who come to the forums or reddit etc..
And then again, if we combine the two theories - you don't actually need to be 1 out of 1.000 who has really bad luck. If you are only a bit better than your ELO and have a tiny tiny bit of bad luck, it's enough to counter that bit of skill you have. Imagine you are in Silver 1 and want your Victorious Skin and you have the tiniest bit of bad luck - oh dear lord will you be mad about it.
1 out of 10 players that is slightly lower in divisions than his skill will have this tiny bit of bad luck. This means 9 out of 10 will be in the correct division of course! However, with 30 million players you can already see that there is MANY who will suffer from this effect.
Let's not forget that most players don't play 2000 or 3000 games in a season. Without really high game count you can't really eliminate luck. And as I said before, the ones with less luck will be the ones complaining about ELO Hell.