You can go from Fistere to the coastal Portuguese route but is very complicated to find your way and it is a long way so better is to return back to Santiago and follow the blue waymarkers to Fátima in Portugal . So you will arrive in Porto.
You can cross Gallicia to Pontevedra It is a road along the Galician coast to Noia and then to Pontevedra were you'll find the blue waymarkers
Then head to Redondela and then to Vigo. Once on your way from Vigo to Baiona keep the ocean at your right and as long as the ocean is at your side you will arrive in Porto.
I travelled a lot trough Galicia and the north of Portugal and made the suggested trip by car Very nice trip From Fistere to Pontevedra was about 140 kms by car