Is your internet connection Optus Cable or Optus NBN HFC? If its NBN HFC, have you tried connecting the work laptop directly into the NBN HFC box?
The work laptop will no doubt be using VPN software to allow remote secure connections to the work servers which may require port forwarding to be configured on your Optus modem/router. Has her work provided her with any troubleshooting information or anything that talks about "port forwarding", internet configuration, or anything like that?
The "will only stay connected for a few minutes" is one indicator that port forwarding may be required.