KPN Store Amstelveen promised I could change my promo deal

Beste KPN Moderators,
I am writing this post because I was misinformed by a sales employee at the KPN Winkel in Amstelveen (Stadshart), and I need your help to fix my home internet contract status.
When I signed my contract in the Amstelveen store, the salesperson explicitly promised me a 30-day grace period (bedenktijd) to convince me to sign up. Based on this verbal agreement, I signed the internet  contract for 24 months.
My internet line was self-installed and activated early on May 15, meaning today is day 16. However, my official welcome paperwork states an internet start date of May 18 (which means I am only on day 13 of my official contract start).
I want to use the 30-day grace period explicitly promised by your store employee to switch my current subscription to the active promotion: the Philips Ambilight 55-inch TV deal.
When I contacted customer service and visited the store, I was blocked because the system says the 14-day window passed (calculated from the early May 15 activation). This is unfair because:

   1. The store employee verbally bound KPN to a 30-day trial window (dwaling/misleiding).
   2. Even by the official paperwork start date (May 18), I am still within 14 days.
   3. KPN can easily check their internal systems for my store transaction to see which employee ID code handled my registration at the Amstelveen branch.

Can a moderator please step in, contact the Amstelveen store manager if needed, and help me adjust my contract so I can claim the Philips Ambilight 55" TV promotion?
I have filled in my customer number and address details in my forum profile settings so you can look up my account securely.
Kind regards,
Venkata Satyanarayana Pilli