Not sure what you're referring to (round part in the middle). I suspect there is a pronounced bead lock ridge. Breaking the bead on a well seated tire can be a real pain and takes a LOT of force. I have used the loader bucket cutting edge to push down on the tire sidewall right along the rim, driven heavy vehicles over the sidewall of the tire and done some serous pounding with a sledge hammer and bar directly on the bead to get them to finally let loose. You will have to be inventive on how to apply some serious pressure directly to the tire bead but be careful not to use something that will tear or puncture the sidewall. You may be forced to take it to a tire shop and have them break the bead for you.....no shame in that and not very expensive.
Sorry to have sounded so basic in my first response but you never know how knowledgeable/skilled someone is. The safety police will probably frown but I wouldn't have even used a jack as I first described. My buddy showed me years ago that most skidsteers are built so you can put the bucket flat on the ground, put blocks under the back of the skidsteer and then curl the bucket down like you're dumping and it'll lift all 4 wheels off the ground....quick and easy.