5e is not set in the Forgotten Realms. As with any other D&D edition out there, it is set in a world of your choice, be it an official world like FR or PoL, or in one you created yourself.
It just so happens that the first bunch of offical adventures have been set in the Realms, and that the developers chose to have FR as the default setting, because it offered them a good swath of lore and enough diversity to be easily adaptable to many existing adventures that were not set in a definite world.
This means that you could just go on playing on PoL, while maybe adding some FR cults and organizations, just as if they always existed on that setting. PoL is a world with a lot of wilderness and who knows what hides behind the next hill.
In the same way, you might set PoL somewhere in Toril, not necessarily near to Faerun. Maybe you're deep in the woods of Maztica, and the people of PoL are the descendants of a bunch of Amn colonizers who traveled to its far west.
Another good way to merge the two settings is by opening a portal between the two worlds.
Note: I heard Maztica disappeared between 3rd and 4th edition. Maybe it returned to its place in 5e, maybe not, but you'd better check (if you love consistency).