Welcome. You can’t trunk to a SIP provider without their cooperation. If they won’t give you SIP connection details, it’s unlikely anybody here will be able to.
Does the modem provide dial tone to the telephone port(s)? If so, you could connect the FreePBX server to the phone ports on the modem if you have the necessary hardware (TDM card or gateway with 1 FXO port per phone port on the modem). Even if it does work, it’s a horrible solution, you really want a provider that allows you to trunk directly without putting analog devices in the middle.