Yes, I can certainly help you with your Apache setup - and based on the information gathering that I have conducted on your domain name (that you provided in the initial post) as well as open-source intelligence that I was able to gather, I believe that you are working with a misconfigured Apache server and that this is what is preventing you from accessing your Mattermost instance.
Here is what I have found, and how I interpret this information:
First, I simply opened the original URL that you provided and attempted to access it, which of course failed.
Second, I used my Kali Linux server installation to run a quick reverse DNS query to the domain name you provided, which gave me the origin IP address of your server, as you can see from the below output of the reverse DNS query:
root@kali:~# dig chat.kozakis.net
; <<>> DiG 9.16.11-Debian <<>> chat.kozakis.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31441
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 4
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;chat.kozakis.net. IN A
;; ANSWER SECTION:
chat.kozakis.net. 248 IN A 208.113.166.163
;; AUTHORITY SECTION:
kozakis.net. 172351 IN NS ns3.dreamhost.com.
kozakis.net. 172351 IN NS ns2.dreamhost.com.
kozakis.net. 172351 IN NS ns1.dreamhost.com.
;; ADDITIONAL SECTION:
ns1.dreamhost.com. 284 IN A 162.159.26.14
ns2.dreamhost.com. 284 IN A 162.159.26.81
ns3.dreamhost.com. 284 IN A 162.159.27.84
;; Query time: 23 msec
;; SERVER: 72.240.13.5#53(72.240.13.5)
;; WHEN: Tue Feb 23 16:23:21 EST 2021
;; MSG SIZE rcvd: 176
The line below is the line that contains the origin IP address of the webserver that you are hosting your Mattermost instance from. (Correct me if that is not the right IP address, because then there are even bigger issues than those I have identified here)
chat.kozakis.net. 248 IN A 208.113.166.163
To make sure that this IP address is indeed the origin IP address and not a web application firewall’s IP address, such as a Cloudflare IP address, I used a service (can’t specify which one for client confidentiality reasons) to return the following JSON array, confirming that it is the real origin IP address:
{"as":"AS26347 New Dream Network, LLC","city":"Brea","country":"United States","countryCode":"US","isp":"New Dream Network, LLC","lat":33.9119,"lon":-117.881,"org":"New Dream Network, LLC","query":"208.113.166.163","region":"CA","regionName":"California","status":"success","timezone":"America/Los_Angeles","zip":"92821"}
Now that I confirmed to myself that the IP address is indeed the correct address to investigate, I ran a port scan (again from my Kali Linux machine) to determine what services are open on your server. Here is the result of my first scan:
Host is up (0.062s latency).
Not shown: 65532 filtered ports
PORT STATE SERVICE
22/tcp open ssh
80/tcp closed http
443/tcp closed https
First, this indicates that the port 8065 which you are attempting to connect to by appending :8065 to the end of your URL is not open, and therefore cannot be connected to regardless.
Second, unless there is an enhanced security feature that is blocking direct requests to the IP address, your firewall is currently blocking all external requests to this domain name, which is obviously not going to allow you to connect to the website. This can be changed by modifying your firewall access rules to allow traffic on both the HTTP port 80 and the HTTPS port 443.
Third, as a note on the configuration, you need to use Apache as a reverse proxy with Mattermost. This allows you to access Mattermost from chat.example.com instead of having to navigate to chat.example.com:8065, and also significantly improves the security of your Mattermost instance. This is going to require rewriting your Apache VirtualHost configuration file, which I am more than willing and capable of assisting you with.
Finally, you are going to want to use HTTPS SSL encryption on your chat service and as such, it would be useful to install CertBot which is free, on your server.
Does this make sense to you, and would you be able to further elaborate on the Arch and operating system of the webserver you are currently using?
Thanks! 
) I’ll be able to easily help you set it up correctly!