Hi,
First of all, check if the IP they get is the right IP, you might need to enter an Custom APN in Network > Mobile page.
If the problem persists - check if the device you're connecting with doesn't have a static IP set and uses DHCP.
If that's only one SIM card that is not working - try another device. If another device acts the same as the previous one - try changing the SIM or call the operator and ask if everything is enabled within that SIM service.
EB.