Is it permissible to use a phone without my father's knowledge for studying when necessary, and am I sinful for giving private lessons to pay off debts, also without his knowledge?

There is no harm for you in using the phone without your father's permission for permissible and beneficial purposes, because obedience to the father is obligatory in matters where he has a valid interest and there is no harm to the child. No valid interest appears in preventing you from using it for such benefits. There is no harm for you in working as a teacher to earn money and pay off debts.