How to use mobile data (hotspot) and calls at the same time?
Going by your problem I suspected that your device is Dual SIM Dual Standby, which was confirmed by GSM Arena.  

What this means is that you can use only one SIM at a time be it for data or voice unlike Dual SIM Dual Active phones where you can use both SIMs simultaneously for voice /data.
Also the user manual which can be downloaded from here does not mention Dual SIM Dual Active. 

So, contrary to your expectations you have only one modem and hence the behavior you experience. 

Only solution is to use one SIM that has both voice and data plans since your device supports VoLTE (see user manual). Be aware that if you use both SIMs on 4G/VoLTE, only one will work on 4G and the other on 2G