I don't have this phone, but the rules are similar for similar devices. You can save contacts on the SIM card or in the phone memory. You probably write to the SIM card, but the memory is very small there. You can try to transfer contacts to the phone's memory or save new ones in the phone's memory. How to do it? Check your owner's manual, and if that doesn't help, contact your dealer or manufacturer for help.
I'm afraid they don't provide this information. The phone should definitely support an 8 GB memory card, but possibly even 32 GB. My advice: please check practically before buying. You can borrow a card of a friend or someone from your family for a moment. Alternatively: contact the manufacturer's support: https://www.doro.com/en-us/support/
Good luck!
Good luck!