Cod.
I think that is the motivation needed. We have some pretty good evidence that Europeans were fishing off the coast of North America in the decades before Columbus arrived. Ancient Greece imported salted fish from the Black Sea. So the POD is pretty easy. At some point in time, an Emperor or other high Roman official pushes fishing for food the ways North Africa was developed for food for Rome. POD can go a lot of ways, but just to pick one.
Augustus encourages imported of salted fish from Atlantic to feed masses of Rome. We add a little fish to the bread and circus type ideas. I believe the distance from fishing grounds near the UK to Rome (ITL) in time is about the same as from Eastern Canada to the NW Spain (OTL). Idea goes ok, but not great. Around 120 AD, some emperor decides to built large imperial city near Rhine to help control the border. Say it is near modern Antwerp. These fish supplies are divert to new city, much like grain of Egypt was diverted to eastern empire IOTL. Over the next century, this city grows in size. As does the size of the fishing fleets, as does the size of the ships. These ships routinely go as far as Iceland to fish by 220 AD. By roughly 200 AD there are three main cities of the Roman Empire. Antwerp, Rome, Constantinople. By 250 AD, these ships are fishing off modern Canada. By 270 AD, these ships are in large numbers.
Now there are massive butterflies from OTL, but you can take this ATL lots of ways, and many of these ways have decent size Roman ports on the coast of North America. My view is that generally speaking, given a strong motive, basic technology can be moved backward in time.