Hello and congrats on the new puppy:D
I got Krose when he was 3,5 months old. He was potty trained either but he learned so quickly and after the first week, there were only a few accidents in the house and in 2 weeks, I must say with my help too, he was potty trained

I always wanted him to feel he did something outragious and gave him treats and praised him. After the first week, I didn't make him do his business in the house ever!!!
I would take him outside every 2 hours or so, so I'm thinking maybe your 20 min. is a bit too often?
Every 2 hours worked for me and Krose wonderfully and after 2 weeks, He would go out in the morning, at noon, in the evening and at night before going to bed. That is still his schedule. Some of my friends think it is too often but I don't think so. If he has the chance to go out, why not give him that?
Anyway, I just wanted to share my ''potty training'' story with you
Good luck, and remember that this will be over soon:D
basak