Changing rear keyless entry antenna

[makan.dm]

I have a 07 cx9 grand touring which fibs on the doors don't work for locking and unlocking, and red KEY sign is on the dashboard. They told me it's probably from rear antenna. I bought one and plug it in but still sign is on and doesn't work.
What should I do? Please help
Thank you

 

[ceric]

Who are "they"?

 

[makan.dm]

Machanics. The code was B1131