I have not GS UCM ip pbx yet.
Grand Stream GXP1625 Date / Time settings always be resetted after reboot
You could connect 1 device to this network if ITSP allow (with NTP), worth ask.
if you set a date and time manually from your phone after every reboot all of date and time reset to factory as you said .
you should set ntp server or set time to self define and set time zone of your country manually then save and apply .
in this way after every reboot date and time will be OK.
I gave this problem can I make my issabel box as NTP server as we’ll or the grandstream phones when set manually can keep time correctly after a reboot