What kind of SSL certificate I need?

Hi guys, 

Let me introduce myself first, I am a Freelance IT, I setup computers, cabling and help users with common problems at office or with printers as well I can take care of servers hardware.

Actually one of my clients is a branch of the foreign affairs of my country, the security for them is very important as you can imagine.

I have a small linux web server installed at home, there I have a web ticketing system which I want to use to keep a fast contact with users and give a fast response and solution when they have problems.

From outside the office I can access but not from inside network, it seems there's a problem with the configuration of the SSL, I have few domains in that server, 2 of them with free SSL certificate provided by lets-encrypt and the other one was the cheapest I found just to try but with the same results (ComodoSSL).

In any case when I try to acces to such domains there's a message "timeout" or if I try from my smatphone "ERR_CERT_COMMON_NAME_INVALID" ) always from inside of office network.

From an external network I don't have any problem.

I tried to talk with the sysadmin, but it seems he didn't understand what I was talking..so I cannot count on him.

By the way these domains are setup with Google DNS dynamic domains (who knows maybe that's the problem) and the server is a Linux Debian machine with Apache.

So did you have a similar problem as it?

The address of one of the servers is www.trackoffers.net  I used in the past in another company don't worry there's nothing.

Kind regards