|Your laptop will have some sort of procedure to turn on the embedded numeric keypad.|
For example, to enable the embedded numeric keypad on my IBM T23, I press shift+scrlk.
So the sequence to type ° for me is:
Shift+Scrlk, Alt+248, Shift+Scrlk to return to normal keyboard.
I think in your case it's
Numlk, Alt+248, Numlk to return to normal keyboard.