What could be the problem (No access to the application)?

Question

linux

There is a VPS with Centos i686 x32. I start a service on it that works via the UDP protocol on port 7777.
The service starts successfully (within the control interface for root), namely:
There are no errors in the service log.
There are no errors in the launch environment log (nohup.out). In the top
utility , the service is displayed, consumes the proper amount of resources. When trying to connect to this service through a client on a remote user machine (PC), no response is received from this service. It does not seem to exist on the Internet, although the service itself should be actively visible on the Internet. When checking 7777 port udp request via nmap:

# nmap -sU -p 7777 111.111.111.111 // IP для примера

It gives out that the port is "open/filtered", but everything is fine here. The fact is that I ran exactly the same service, on a very similar x32 distribution, also CentOS, on another VPS, and there it is clearly visible on the Internet. I specifically, identifying the problem, took advantage of another VPS. There, all the "rings" of the ports gave the same result.
In addition, I disabled iptables:
# service iptables stop
But the result did not change.
Then I turned to the hosting provider, they said that UDP is not blocked and the matter is in the service settings. Although I run the service with the same settings on another VPS, and it should work.
And here I have some versions, but I hope you will add them.
It is possible that the problem is in the distribution, although there is almost no difference between the distributions of the two VPS. And there and there x32.
Well, with a small probability, I think the problem is blocking traffic. Although I even disabled iptables, and the hoster does not block anything. And even checked for special. port sites. Writes that it is open.

Answer 1

on the server:
knock on the client

1. you see incoming packets, but no outgoing ones - the problem is either in the firewall, or in the application, or in routing
   
2. you see a two-way exchange - a problem in addressing responses
   
3. nothing arrives - an obstacle in the pass (perhaps small UDP packets are filtered out)
   

Answer 2

netstat -lu says what?
selinux policies configured?
try disabling SELinux for testing - setenforce 0