Firewall Zero Hour is a great game, however it is also plagued with great problems, the main problem for me (and many others) is that it is peer to peer based rather than server based.
One player is the host and other players connect to them (host may have a slight advantage with latency), if the host loses connection or quits/rage quits then everyone is back to the lobby.
I'm sure there are lots of things that can cause connection problems but a redditor has pinned down something that has been causing them trouble, check out their post below.
Essentially the users phone is using a lot of upstream bandwidth and is causing frequent drop outs which doesn't seem to occur when the phone is in airplane mode.
This seems to be phone specific and may be due to an android update.
Having your phone off when playing isn't an optimum solution so the redditor dug deeper, some routers have a feature called QoS which allows you to assign bandwidth priority to certain devices, with one of these routers to hand and priority bandwidth given to the PS4 they had a much more stable connection.
So if you're having connection issues in Firewall Zero Hour or other games, test it with no other devices on your network, if it works better then try a router that supports QoS or add your devices back one at a time until you know which one is causing the issues.