Nighthawk M1 Mobile Router

NTG9X50C_12.06.12.00

I am operating this unit as a modem only ( DHCP off, IP Passthrough on ) connected to my Apple Airport Time Capsule and Vodafone UK LTE network.  Everything works fine, although the speeds are somewhat less than I would expect.  The orange light on the modem flashes once every four seconds.  Does this indicate that the unit has a problem communicating with the mobile network or with my router?

If I turn-off IP Passthrough the light turns blue but I have no internet access.