Outside Potty Training (Mini Dachshund)

Hello all- I apologize if this has been discussed before.

I have a 8 month Mini Dachshund, and potty training outside has been very hard.
The problems is that the only place the dog will go potty in our small backyard on a strip of fake grass. She will not go potty pretty much anywhere else other than that strip of grass.

She has never gone potty when we take her on walks or to parks...and not for lack of trying. We take her on walks often and give her verbal instructions "go potty" when we get to grass areas, to no success. She will hold her potty no matter where we are, until we get back to our house and she can go to the strip of fake grass in our backyard. We have been to parks for 2-3 hours before and she has never gone potty while there. We have never been able to get her to go potty while on a leash one single time. She's has been to our in laws house which has a big back yard full of grass, and she's held her potty for 6+ hours MANY times until she gets home.

Please help