If you check the release notes for firmware version 12.3.4 you will see a different boot ROM image must be uploaded to the unit first, or it will not be able to boot at all. Contact support and ask for a link to download the new boot ROM image 17.01.01.B2 (corresponding to Netvanta 3120 part #1700601G2) along with instructions for flashing the device with it. If you need Adtran Support to do the software upgrade for your customer's unit, you will have to pay for a support ticket.
Hope this helps.
Thank you for the reply Mick. I'll get in touch with support then, regarding flashing a new boot ROM image on the 3120.
All the best,