In Fly:Manual, be aware that you will still get GPS lock later (in the air, away from your takeoff point). When this happens, "Home" won't be where you took off from. Don't use RTH. Be aware that if a fail-safe (low battery, lost connection, etc.) kicks in, your Solo won't RTH to your take-off point. If RTH does kick in, just press the "Pause" button and take control again and land manually.
Also, Fly:Manual is also known as "Altitude Hold". The barometer will try to maintain your altitude, but there's no GPS control to maintain roll and pitch (a steady location). If you let go of the sticks, Solo will stay in the air, but will slide sideways and glide with the wind.
But this doesn't solve your initial problem. Why are you getting a high HDOP and can't get a solid GPS lock? Be sure to try in a very open area away from buildings, many trees, and mountain sides.