You bring up a really good point. I believe that Angstrom was aware of Cecil, since he said he was searching for someone to take on the mission of getting the machine to work, and only the Maulers had the necessary knowledge. It’d be weird if during his search he didn’t stumble upon robot, or some other figure connected to Cecil, or even Cecil himself.

As for whether Cecil would believe Angstrom or not, doesn’t really matter because of his powers. If he did ask Cecil, there’s a chance he believes him and helps him. If Cecil doesn’t believe him, Angstrom just opens another portal to a different dimension slightly different from the last one and it’s like he never asked Cecil in the first place. Though now that I’m saying this, maybe he did do that and that’s why we don’t see him trying to contact Cecil in his current dimension at the time. Fair enough