Hello Shawtay! there's 2 things needed to enter to Fenrock Dragon Lords:
You need to have started The Hidden City of Beregar Quest to be able to access Fenrock. and the second thing is a Golden Mug. Note that you need one Golden Mug for every time you enter the dungeon. And last but not less important be level 75+
There is two ways to gain access to this Dragon Lair:
By as well as to gain access to Beregar and use the Ore Wagon shortcut.
Or walking all the way from the entrance of the island to the Dragon Lair, following the images below:
Actually there is no a level needed to enter into the Dragon Lair I would like to recommend you 80 for mages and 100 for RP's just because the way is very long and on a less level the refill will be not worth. There is only needed level 100+ to finish the last part of the Hidden of Beregar Quest where you have to open Pythius the Rotten chamber.
I made my own test on 2 characters one level 50 and one level 75.
The level 50 wasn't allowed to enter, and the level 75 could enter without problem.
I'm afraid to say that the minimum level to enter is 75, cuz maybe ever lowers can enter but as I tested makers lvl 75+ can enter without problems.
First of all you must travel to Mistrock and once you get there you should ask to Maris about "Mistrock and right after you must say "Fenrock- Yes ". Once you are at Fenrock you have to go to the to the -2 and use the pick, right here:
It's very important talk with Maris first because if not, you will be unable to open the hole.
I followed this way of the pic taken from Tibia Fandom, that ride me to the NPC Pythius the Rotten:
Once you have reached this NPC you must say: Hi - offer - yes - Golden Mug. After that, you will be able to go through the TP and finally you will reach the cave at the Fenrock DLs: