I have a XZ Premium purchased in August 2017 from Singapore. I currently live in Mumbai, India. I'm on the Vodafone network for the past year and over time the 4g network speeds git so slow, even Google browser doesn't open 90% of the time. I can't use maps, Uber, anything that requires basic mobile data connectivity! And friends with lower end Huawei, Xiaomi, Vivo phones mock my Sony as they're able to use their mobile data with minimal/no issues.
I have raised this issue with Vodafone several times and even changed my sim card. No difference whatsoever.
Have you tried with a SIM card from a different provider in your device? Just to make sure that your current operator isn't the cause of this.
Since the device is purchased in another country, check which LTE bands your operator is using. Below I have listed the supported LTE bands for the different XZ Premium models (Settings > System > About phone > Phone model).