It is correct that priority is not to walk more than a certain distance if possible. However, if necessary, one is allowed to wash under the same roof.
There are different opinions regarding whether or not one must wash within that distance (4 amos) if it’s possible or if it’s okay to rely on the “anywhere under the same roof” even when one had the option of setting up negelvasser beforehand. That’s why there are many people who set up negelvasser near their bed the night before, and many do not do so.
I didn’t mention it in my prior post since: 1. There are many who hold that it’s okay to wash under the same roof even if you have the option of setting up negelvasser the night before, & 2. In your case, where it was not an option, it should certainly be okay.
However, if this is something you are normally makpid (careful to keep) about, then you should probably ask a sheilah about what to do in this case. You have mentioned Chabad, and I am under the impression that Chassidim may be more stringent in this manner.
When you hear back from the Rabbi, I would be interested in hearing what he says. Which questions did you ask him?