several domains, only one is responsive on smartfones

[j36g2]

hello! my wordpress domain noorden-capital.com
i designed it responsiv (only smartfone mode, but is also showing on tablet formats)
my client wants the domain to stay noordencapital.de
OK, i installed the forwarding on my hoster system.
BUT
it only shows responsive design on the noorden-capital.com on the smartfones, not on the other noordencapital.de domain.
where can i search for the problem?

[arminunruh]

dont use the iframe way of redirecting

use the redirect where if you go to noordencapital.de, it will redirect you to noorden-capital.com so the URL in the addressbar will actually be noorden-capital.com

[j36g2]

ok, this is not what my client wants, but ok, i have to find another way

[arminunruh]

i cant really think of another way :/