Hi
a way to reset nRF9160 is by pulling low the nRESET pin https://infocenter.nordicsemi.com/topic/ps_nrf9160/pmureset.html?cp=2_0_0_4_0_4_1#pin_reset
During the entire reset process, can you confirm that VDD and VDD_GPIO can be kept High as normal? (basically, that no action what so ever is needed on VDD_GPIO and VDD)
Thanks
BR
LA