Basic-Fit complaint: misleading information about contract

I had a flex premium membership( 29.99 9 euro extra monthly) since november 2020 to july 2021 and i wanted to change to flex comfort (19. 99 9 euro). I asked a member of staff at the gym and they just told me to go on the app and do it myself, as it is easier instead of them helping me. They also told me i can do it anytime and any difference in price will come back in my bank account. On the app I changed the memberships, but there was no option to apply for the flex part of the membership. Given that this was the instruction i had received from one of your staff members, I thought it would just transfer from my last membership.
However, after contacting customer service, i have found out that i am not able to get a flex membership until july 2022 because i changed it myself from the app instead of cancelling my membership with a 30 days notice.