it says on the modpage compatible with nullspace but in the end nothing else but the center island is there.

am i doing something wrong?

yung's better end island 1.20 forge 2.0.5
nullscape 1.2.4

i tried the reset command which resets the center structure but there is nothing else in the end but that island.

I'm unable to recreate this issue. With the mods listed, everything works as expected, with the rest of the non-center-island terrain being about 1000 blocks away. Killing the dragon and throwing a pearl through the gateway also properly teleports me to the outer islands, so I'm not sure what your issue could be.