Hi @avin123,
I'm afraid that I can't answer for how other devices may behave in regards to network settings, but when you set a preferred network to strictly 3G/WCDMA in your Xperia XA Ultra without your carrier plan supporting it, I'm afraid that 3G will be unavailable.
If your device only supports 2G, then I would recommend setting it to 2G/GSM only, alternatively you could try setting it to 3G (preferred)/GSM. This way, it will try to use 3G if available, but revert to 2G if a 3G connection is not available.
As previous users mentioned in this thread, it would be best to get in touch with your carrier regarding your connectivity concerns. However, I would say that your device functions correctly if it only allows a 2G connection if that's the only network your carrier plan allows.
Feel free to keep me updated regarding this! 
