Sorry I will not rewrite it, this is from the discord messages I sent to aternos support chat. While I didnt get help there, I found this mod to be the caouse of the problem.

Hello! I am playing on an exaroton server with my friend. We are playing with a lot of mods (about 120) and we play on 1.20.1. We have played on this server for a while, and it was bit laggy it didn't bother us much. But today we got on and wanted to explore but the server doesn't load new chunks for its life. ( I created a single player world, absolutely 0 issues runs like butter) But as the server was only on 6 GB RAM, we decided to upgrade it to 10. But nothing changed it still won't load chunk and while it loads the ones we have previously been on, it loads those slow as hell too.

For anyone wondering: The issue was the mod Untamed Wilds. How I found out: As I mentioned I made singleplayer words with these mods with no issue so I thought the problem must be the server. But as I have run out of options I decided to download the world file and look into it more. But as I was in single player, it didnt recognise my character, and it put me at the starting spawn location. HUGE LAG. And then I turned around and the sea is PACKED with untamed wild creatures. I press F3, Entity count 14000.... I think that is pretty self-explanatory. I deleted the mod from the server, and it runs smooth once again!