Hello Dough ,
From 10.4 version we can directly upgrade to 12.1X46 version since its Extended EOL version , so we can Junp 2 EEOL versions .
But since its failing from your end , Try upgrade to 11.4 version ( if you have any other device running the same ) else need to open a JTAc ticket for getting the OS since its taken out from support site .
Or you can take a snapshot from any other 210 running 12.1 version and copy it to this device . That will be the simplest way . ( If you have any 
)