The problem you have is that you have never played OSRS, your account has stats on RS3.
RS3 is the game that has been going for 15 years, OSRS is a newer game that started in 2013.
Because you don't seem to have been active in Runescape since early 2011 you definitely have never played OSRS and that is why you find yourself on Tutorial Island.
If you are using a mobile device, you can only play OSRS.
Jagex has not yet released the mobile version of RS3.
You have already found that your login/password will get you access to both game versions but the character you play is created and trained separately.
About your display name:
Jagex now removes display names from inactive accounts and puts them back into the name pool.
While this won't affect your login 'username', your account was inactive for too long and the display name was removed.
You have had to set a new display name to log into the game today.
So I recreated my old school runescape account earlier this year. I was inactive for several months and went to log back in last night to find my account was gone. The name was given away which was mentioned would happen if I was inactive but after creating a new name it put me back on tutorial island with reset stats.