This is not the first time I encounter this problem with ANA flights booked as code-share (UA).

I have a booking on an ANA flight booked under a code-share number.
If I call ANA to get seats assignment, they keep telling me that because it is under a non-ANA number, I need to contact the other airline.
Of course, as anybody would expect, the other airline cannot assign seats and says to contact ANA.

Does anybody know a trick to break this unhelpful cycle?

PS: I have the ANA booking reference, but the online tool says: *Selected reservation cannot be displayed through the Internet.