1. do the command /locate ruinedportal (overworld or nether)
2. teleport to the coordinates
3. shearch for the portal (use the fill command /fill ~20 ~5 ~20 ~-20 ~-5 ~-20 air 0 replace <Netherrack, lava, grass, stone, water or gravel ect...>)

you will see that the portal is about 10-20 blocks away from the spot the locate command indicated.

the locate command should be direclty on the block the portal is (a corner of the overall structure at least)

you wil