How to unblock a blocked toilet?
I would use a toilet snake as @Wayfaring Stranger suggested. The home depot treatment you looked at is not a drain cleaner but a septic treatment. You might try warm water. When the water is low in the bowl (as in after plunging), slowly add a large pot of warm water to the bowl. Use warm water from the faucet not boiling water. Boiling water may crack the porcelain. Let it sit for about 5 minutes. Plunge it again. then flush. The warm water may soften soap or grease that is in the trap.