How can I activate another SIM card

Are you based in China mainland, Hong Kong, and Macao? Then you can have dual sim with two physical SIM cards, see this link for instructions

Using Dual SIM with two nano-SIM cards - Apple Support

Further there is no dual sim with 2 physical simcards, but you can use dual sim with e sim. See this link for instructions

Using Dual SIM with an eSIM - Apple Support

Install a physical SIM

1. Insert a paper clip or SIM eject tool into the small hole of the SIM tray, then push in toward iPhone to eject the tray. ... 
2. Remove the tray from iPhone.
3. Place the SIM in the tray. ... 
4. Insert the tray back into iPhone.
5. If you previously set up a PIN on the SIM, carefully enter the PIN when prompted.

Set up cellular service on iPhone - Apple Support (IN)