You need the read the manual for your modem
You can often connect to a modem that is in bridge mode following the user manual's instructions to verify it's settings.
Try connecting it to a PC/laptop and if you get the address 192.168.1.100 try connecting to http://192.168.1.1 (my last modem was like that). There might be an admin interface there. Once you get it working this way, then connect it to pfSense.
You could temporarily receive a private address while the modem is working things out with the provider. I've seen that before. But it should change to a public IP shortly after.
Update: Since you are using PPPoE - many modems just pass along (pass-through) PPPoE now despite how they are configured, so just try connecting if you think it's setup right. Ignore the fact that it also has a local IP. You can't get a public IP address with a DSL modem that is on a PPPoE network without connecting with PPPoE. Once you connect the modem to pfSense, it will no longer hand out the 192.168.1.100 address because pfSense will not query it with DHCP (unless you setup something funny).