First of all you must defeat the elite four and if you have already done that then you probably don't have your national pokedex, to get the national pokedex you need to have seen or caught (which ever you prefer) 210 pokemon afterwards you go to sandgem town and talk to Professor Rowan and he will then give you the pokedex, after that you will be able to access new areas such as route 228, survival area and resort area also new pokemon from some of the older genertions of pokemon.
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In Pokemon Platinum, the player can reach Route 228 by heading through Route 226 to come out on the north end of Route 228.
route 228 is near the resort area on the big island.
You can find it at route 228 with the poke radar
In Iron Island and Route 228.
beside the ribbon house