Obtaining a social security number as a foreigner coming for a PhD

Good afternoon.

I am south african and will start a PhD in France very soon. I am currently stuck in a circular situation as :

• I need a social security number and a bank account for the establishment of my contract
• I need a visa to have a bank account
• I need a contract to finalise my university registration
• I need a contract to get a visa

Am I supposed to register online to a student social security ? The procedures requires a visa (which itself requires a contract)

Am I supposed to be registered by the general CPAM ? The procedures has to be done by my employer who himself requires a social security number ?

Thank you for your advices