Wait. You have a dedicated server with one public IP address. Did I understand that correctly so far? If yes, I'll continue.
I assume "x.x.x.177" is the IP with which you connect to the dedicated server remotely (SSH).
So why exactly are you setting -ip to "172.16.20.3"? I don't get this point tbh. Remove the "-ip" parameter. You don't really need it.
Code:
-nobreakpad -game csgo -console -usercon -secure -maxplayers_override 20 -tickrate 128 +hostport 27015 +net_public_adr x.x.x.177 -nohltv +sv_pure 0 +game_type 0 +game_mode 1 +mapgroup mg_bomb +map de_dust2
^Try these start parameters.