In order to get to Cinnabar Island in Pokemon Let’s Go, you will have to clear the Seafoam Islands. Check out this Pokemon Let’s Go Seafoam Island guide to get through quickly. After that you can go claim your fire badge.
Pokemon Let’s Go Seafoam Island GuideWhen you enter Seafoam Island you will be greeted by Ice. Take the path to the right and push the block down the hole. If you don’t have push yet, we have a guide on how to get it here. Push that block down the hole and then follow it down. Go around the block and the hole and push it down the next hole. Go around again and push it a final time and follow it. This will put you in the water. Get out of the water and return to the first floor(cave entrance). When you return to the entrance, go to the upper path and follow it around to the right. Push the second block up, then get behind it and push it down the hole. Just like last time you want to push it all the way to the bottom. When both blocks are down you will be able to surf freely. Take the right path up and out to be free of the Seafoam Islands.
Articuno is also in Seafoam Cave and we have a guide on how to find it here.
in Pokemon Let's Go Pikachu & Eevee Guides