Hi Aldo,
First, it doesn't seem like you are getting much help from support. This means you may need to get your help from the forum and/or have someone remote in to look at your set up.
One thing about allowing others to help you when troubleshooting is to try to do the things suggested and report back. Their suggestions may be helpful or may not be, but if you skip around, then they don't have the information they can use to offer the next step. I am not sure what the speeds of your hot spot are. If it were me, I would make sure I had a Gb router and with the modem connected directly to the router and only one computer connected to the router, what speeds are you getting. And, what is the download speed and upload speed of your Internet at the office? And, what is the speed of your Ethernet card on the workstation.
As JBS suggested, you can knock your head against the wall forever, but at some point if you are going to continue using AC, you will have to move back to a LAN. It really isn't that difficult to maintain an LAN, and there are plenty of companies who can set one up and even maintain it.
If it has gotten to the point where staff are throwing keyboards against the wall, then it is time to take the next step. Set up a network, ask for your data, and start working again.
