The gateway address of the BMEAHI0810 module should be the IP address of the fieldbus network of the BMENOC0321.
For instance , below the Ethernet network Manager screen of my application where the gateway of my AHI is the IP address of the router :

In the BMEAHI0812 DTM instance, you need to enable the option "Firmware upgrade"  as follow :

And always perform a transfer to FDR server after each modification of the BMEAHI0812 DTM instance.
Then open Unity Loader and connect to the IP address of your BMEAHI0812 module to perform the upgrade :

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