From what I understand, you want to turn SIM2 as default or main if SIM1 fails or there is none connected to this slot, correct? This can be done with the SIM Switch feature. In the link you sent, there is a SIM switch section, there is more information on how to configure this. That is, switch to SIM2 if SIM1 has no connectivity, bad signal, or data limit among others.
What you cannot do is display the SIM inserted in slot 2 as SIM1. You will always have the SIM information concerning the slot where it is inserted, and you can make it main thanks to this feature. However, it will always be displayed in the WebUI as SIM2.