Good idea. Ask them what type of network do they have - is it LTE? or HSPA+? or something else?OK, I'll call my operator (TESCO) again, and ask them what's up.
Edit: do you have the US version or the rest of the world version of Nexus 5? (820/821) The US version supports LTE for the US carriers, but has limited support for the LTE bands used overseas. The 821 has support for more LTE bands used overseas