You need the Armor Pass
So to go to the Isle of Armor, you first need to update your game. After you've updated your game, go into it. Then, you get the Armor Pass. After you've gotten the Armor Pass, then you go to the Wedgehurst station. When you get there, you catch the Galarian Slowpoke, as you have done that. Then you talk to the rail guard standing there, if he isn't there, update your game. Then you talk to him and he gives you an option to go to the Armor Station. Say 'Yes' and you'll be on your way. Good luck!
Hope I helped!