This worked for us:
No puppy pads, no area indoors where it was suitable to toilet.
Taken outside every time he woke up, before naps, before food, after food, before playing, after playing, or every twenty-thirty minutes if none of those things have occurred!
Wait for a pee or poo. Wait. And... wait. At least 10 minutes. When he peed, we quietly chanted his cue word. Done just when he was in the action of peeing. Quiet praise and back indoors or play outside. No treat rewards. The relief of going should be reward enough.
Accidents never commented on (although I did sigh inside!). Whip him out if mid-pee, just so he gets the idea. Otherwise clean it up ASAP with biological washing liquid or a specialist puppy pee spray.
We had Henry clean in the house within a few weeks, but it took a few more weeks after that before he would actively ask to go out. It's all about making sure he's outdoors at times when he might need to go.