Disregard mo yan. Yung 121.54.70.142 is DNS Server ni Smart. Still, I don't recommend na disable mo firewall mo.
From a Security Standpoint, delikado i-disable yung Firewall sa kahit anong network unless alam mo ginagawa mo.
As for 121.54.70.142, well as I said, DNS Server lang yan, it's relatively harmless since trusted DNS Provider naman si Smart.
Smart and PLDT users alike are under what's called a CGNAT. Let's just say na parang Firewall rin ito, as multiple users use the same Public IP as you.