Feniks 3 firmware rollback

Good morning

My old Feniks 3 has just been replenished with new battery after the first did not last longer than a day or two. Unfortunately, the new battery exibits the exact behaviour. I have uninstalled everything that could cause drain, switched off wifi and bluetooth connections, and deactivated my Strava account. Still the same.

Today I tried rolling back the firmware to pre-V9 but cannot get it to work. Each time it restarts, the version is upgraded to 9.6.

Any suggestions?