the physical key (I presume you have the smart key, you didn't say which model you have) is only for entry into the vehicle not to start the vehicle (was the same on my 2016 Soul).
from page 5-15 in the 2021 Seltos Canadian owners manual:
"if the battery is weak or the smart key does not work correctly, you can start the engine by pressing the ENGINE START/STOP button with the smart key. When you press the ENGINE/START/STOP button directly with the smart key, the smart key should contact the button at a right angle."
from page 5-15 in the 2021 Seltos Canadian owners manual:
"if the battery is weak or the smart key does not work correctly, you can start the engine by pressing the ENGINE START/STOP button with the smart key. When you press the ENGINE/START/STOP button directly with the smart key, the smart key should contact the button at a right angle."