I’m not an expert on repair, and my knowledge barely exceeds that of a basic layman, but I might be able to help out a little bit. Your problem sounds similar to an issue I had with my ‘27 Remington Portable when I first got it. It would type fine until I got about halfway across the page and then the carriage would stop completely. I poked around and discovered that the drawband had slipped off its track on the mainspring and had tangled around the screw that mounts the mainspring. Once I had reseated the drawband in its track (and the little guide pulley further along) that resolved the issue; I’ve not had any problems since then.

I’m not that knowledgeable on Portable 5 mechanics, but knowing Remington I would say the setup is probably similar -if not identical- to it’s 1920s predecessor. I hope this can help!