I suspect the replacement of the chain is overdue, a New KMC chain will come with a easier to use Connector link
5,6,7,8 all use the same chain width..

the Park Tool, Master link Pliers Park Tool Co. » MLP-1.2 : Master Link Pliers : Chain is to squeeze the rollers of 2 inside links together .

That is how the current connectors open easily, the opposite of the tension on the chain moving the bike forward..