This is likely due to you switching from using EE to Vodafone for coverage.
Virgin are currently in the process of moving its mobile service from the EE network to Vodafone, depending on mast locations you may see an improvement or loss of service.
The only advice that can be given is either contact Virgin and return the device and look at another network that provides better coverage in your area or to enable WiFi calling service to keep you connected at home when signal is weak or not available.