Welcome!
Troubleshooting steps for collisions
1) Export collision at 0,0,0 XYZ coordinates.
2) Ensure the rotations and scale values are at their defaults. If they aren't, you can reset the values through Hierarchy tab on your right side -> Reset scale/transforms.
3) Ensure the dimensions are less than 256 +/- on each axis, within a 512m cube.
4) Ensure the model topology is clean and has no isolated edges or vertices, as these can cause invisible walls.
5) While a collision can contain 32k vertices, it's strongly recommended to stay below 5k.
6) For invisible walls, install Goldfish's edition of Kam's scripts and run CST exporter. Export .cst -> import into Steve M's Collision Editor -> save as single .col file. Goldfish's Kam's scripts: https://gtaforums.com/topic/907323-rel-kams-gta-scripts-2018/