Give a bit more information on exactly which Decathlon electric you have.

2 years or so back I bought the Wattage "mini" Decathlon EP with a 28.75" and area of 151 sq. in. 15.4-16oz a sq ft.
I know wattage stopped making that plane and came out with the larger EP in a red scheme. Super Decathlon I think they call it.

Here I am speaking of my Wattege mini in yellow and white trim.
Gave that mini Decathlon plane a few tries and got on here asking questions. Many seasoned flyers had also tried to fly this plane. One fellow told me that they passed it around the club and no one could fly it.
To much wing load. My experience was the same as yours. Even the smallest possible inputs would send it into a spin and a crash.
I gave it try after try, attempted with lighter (lithium) batteries, stronger motor. It would fly but no fun at all.
I finally just gave up trying to fly the dog and bought a different electric plane....not by Wattage. That plane turned me off electrics for over a year and absolutly will not but another from wattage no matter what the glowing reviews. Do not trust magazine reviews much anymore and stopped that magazine.

Possibly some have had a great time with that model lately but not back then. I looked long and hard for answers. Only one being add inches to the wing. Not worth the trouble to me.

My Decathlon EP is just a display now sitting in my shop.