Phone loses network connection.

[Kebuda]

I have 2 sim cards in my xperia xz1. The first one is for voice call and sms and the second one is for internet. Sim cards are from different cellular operators. In the same place first cards often loses network connection. If first sim loses connection the second sim loses connection too in the same time. If I disable first sim the second one works stably. If I remove the second sim the first one works unstable. The first sim works stable in other places. The first sim doesn't lose connection in my old mobile phone. How can I solve the problem? 

[Lord_viridis]

Is this in a single specific area?

[Lord_viridis]

Can you swap the Sims in their slots and see if the same occurs

[Kebuda]

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@Viridiswrote:

Is this in a single specific area?

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Yes, I think so. 

>> Can you swap the Sims in their slots and see if the same occurs

Unfortunatelly, it doesn't help. 

[Kebuda]

Upd

I made a bug report (settings->system->developer options->take bug report). There are messages like: 

Fatal error on the modem. 
Modem subsystem failure reason: lte_ml1_rfmgr_stm.c: 8240:assert false failed: cc_id=0 shared_lna_cc_mask=0x0 s. 
Subsys-restart: subsystem_restart_dev(): restart sequence requested for modem, restart level=related.