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Assuming you do NOT have a MBG, in System - IP Settings make sure you have the System NAT IP Address set as your public IP address. Then in Devices and Feature Codes - SIP Peers - SIP Trunk Groups - 92xxx - Configuration - NAT Settings you change the NAT Address Type to Non-SIP Aware NAT.
There are very few things within the MiVO-250 that would require a reset of the system. Typically, a change to the System NAT IP Address does not require a reset to work.