first 3 months 'free' then invoice/debit on day 1

You advertise one can get the first 3 months free, so I go to sign up and see only if I took the expensive router do I have to pay for the first 3 months. I sign up, wait weeks for delivery, then as soon as the courier leaves I get email saying debit order of pro rata plus month 1 will go off the same night.

How is that first 3 months free? Come fetch your stuff please I will go to OpenWeb or another provider who actually delivers what they promise...