Plex Client's does not connect to the local Plex Media Server without internet on the local network?

Hi

I have been having some bad problems these past few updates/months, regarding Plex, this including the PMS, Windows App Store Plex, Android phones, a LG Android TV, and a Samsung TV.

A bit of Background on my setup, I stay in a community where we share a lot of media, and I have a decent Med-High end server, with 10x 1GBs NIC’s setup as link aggregation, and we decided to centralise the media from one point and share it for everyone who contributes on the media and keeps the library updated.

I have internet access on the primary server which is running Windows Server 2012 R2 (Also Tried 2016) I also went as far as reloading the server with Windows 8 and 10 to see if it is an Windows problem regarding the PMS.

Ok so here is the breakdown of what I found, which unfortunately won’t be an option at this current stage as I only have a small internet CAP LTE internet access Dongle, and there is no foreseeable uncapped (without the FUP) internet connections available or being planned (been trying and researching this for almost the past 2 years) that we know of as we are to remote.

I need to run a server, either Virtual or dedicated for the following services: DHCP (For Tracking client connections and Deployment) WDS for Windows installations from the network side, as well as WSUS, For the Windows Updates.

I did setup home users, for each client, with their pin codes for restriction purposes (child friendly etc) as well that each client can keep their watch records.

With the clients, I did setup a temporary internet access (Routing & Remote Access) to the internet on the local network to be able to download and install the latest client versions, but I cannot keep the internet shared. All this worked perfectly until the last few server updates (Which did not have any internet access on the local network). I cannot afford to setup the internet access on the local network as it handles about roughly 50-100 clients dependant on weekdays and/or weekends. and it kills my internet cap of 20GB within less than a day. And the moment the internet goes down, NONE of the clients can see the PMS server what so ever, even with the secure connections disabled/Same Network/Required/Preferred as well as each client’s IP address specified under “List of IP addresses and networks that are allowed without auth”, and “LAN Networks” tried it with setting the individual IP addresses and even with the IP/Subnet config of:
10.0.0.0/8 as well as 10.0.0.0/255.0.0.0.
10.0.0.1/255.0.0.0, 10.0.0.2/255.0.0.0 etc
10.0.0.1/8, 10.0.0.2/8 etc none of which makes a deference.
The moment that I disable the internet sharing from the local network (although the internet is still live on the PMS server, the clients that was not already logged on loses access to the PMS server completely, even with the manual server IP configured, when I disconnect the PMS server from the internet the problem persists but the PMS server can only log in with the last user that logged in from a client.

Where can I start looking to resolve the problem with regards to getting the clients to connect to the PMS server without internet access on the local network which the clients are running the client apps?

I am at a standstill, I am also considering moving over from The Microsoft Server to UnRAID Server OS, but I am not sure if that would solve the problem at all.

The amount of DATA that needs to be transferred including the metadata is just over 20TB, it will be a lengthy process (which I do not mind to undertake) I am also not sure if I can copy and paste the backup of the Plex Media Server over to the UnRaid Plex Media Server Folder for all the already downloaded metadata (about 175GB folder size).

Any advice or feedback would be much appreciated.

[Edit}

I tried now with reloading my intel NUC5i3RYH with Plex Media Player embedded, setup the accounts with internet, and Wireless connections, to the Local Network, and this does seem to go work with no problems what so ever, after I signed in with my Plex Account, setup all the settings and manual server settings, and signed in with one of the users, I disconnected the internet connection completely, restarted the NUC and it still works like a brilliantly, but my concern is, I have an 2 TB SSD’s in the NUC, and it does not give the option of syncing content to the unit, and with the added 16GB of RAM, it is an complete overkill for just an Plex client.
I must admit this worked better then what I expected, especially that the internal IR works great with the XBOX 360 MCE IR Remote. But to convince everyone that connects to the PMS server, to convert to an NUC, is going to be very difficult as I can already understand their point of view, of why need new hardware (OR Reload their current systems) with Plex Embedded, if the client application is available on their devices which was made by Plex for Plex, etc.

Please guys, any assistance on this matter would be much appreciated.