Jonathan, I don't think that's what the OP is trying to do. I think he's trying to print from the hallmark app and it's not printing, so the OP has assumed that WiFi is breaking. As you said, software normally doesn't break WiFi, so that's not it. I think we need to find out if the software will print properly at all, so hardwiring the printer and trying it will either eliminate the software or show that it is a bug in the software. And I don't think the OP wants or needs to attach the printer to the router unless the OP is looking to make it a network printer, which has not been stated. And from the misuse of the term "modem" instead of router, I suspect that rayben is not terribly technically savvy, either. So we need to do this one step at a time.
For Rayben, interference can come from other rooms, other floors. What is beside, above, below where the router and Mac sit?