Workplace network Help!

Hi all, I’m hoping someone can help me with an office networking problem. I’ll preface by saying I’m a Veterinarian and while I feel I’m good at medicine, networking is not a strong suit. I do have moderate computer experience (have done a few builds, can replace motherboard capacitors etc), but I’ve been pounding my head against what I assume is a basic oversight at work for a couple of days.

Scenario: Out veterinary clinic was built In the early 90d and has a dozen workstations or so. We have a wired network for the workstations to access our practice management software and printer off a central workstation. This has never been connected to the internet, mostly to avoid any chance of compromising our client data and of paranoia that we would be more likely to experience crashes with the workstation from employees using/downloading things without knowing what was “safe”.

In the last few years we’ve found the need to add internet access to individual work stations to access email and help clients place online orders at out front desk. I’ve run Ethernet cables to our separate wireless modem (not connected to the offline network) and allowed access individually to the workstations that need it. I’m trying to do the same thing with a workstation at our front desk and having issues. To start the computer (running windows XP professional) only has 1 Ethernet port, which is occupied by our offline network cat5. I’ve had this issue once before, and purchased a usb to cat5 adaptor and that worked no problem (newer workstation running windows 10).

I installed the drivers and connected the 2nd Ethernet cable via the adapter. If both networks are enabled only the first one to connect works... If I enable the one connected to our internet, and then enable the offline network connection I will have internet access but am unable to load our software from the central “server” computer. If I enable the offline network and then internet the opposite happens, can access software but not internet.

Each connection works totally fine when enabled by themselves, but not together. I changed the IP ranges for one of the routers so avoid any overlap, but that didn’t help.

I’m totally stumped and lost here, and guessing I’m overlooking something fairly simple or not understanding a concept all together. If anyone is willing to give me some advice I would be very grateful. I’d also be happy to offer you back companion animal pet advice in return!

Some other specifics- the offline network is connected together via a Cisco router with internet disabled, and just used to assign IP addresses to the computers and printers.

The online network uses an Apple airport router and switch to allow multiple computers to access internet (we have our lab equipment, radiology suite etc all hard wired in).