Hi hchan
The newtmgr
referred to on this forum is a modified version that works with the nRF9160 Feather and Thing Plus nRF9160. You may have to download the official one or mcumgr
if you’re working with Zephyr.
The instructions here only work if you’re using NCS.
Best to get in contact with U-Blox since they’re the best resource for this.