Hi Matthew and Welcome to Digi Forum,
From your description, I understand that the allowed computer is connected to ETH 0 and the SSH accessible device is on ETH1. If that is the case, you should enable the MAC filter on ETH0 as this feature will apply on incoming packets on the interface where the filter is enabled (not for the outgoing packets.
From the User Guide (https://www.digi.com/resources/documentation/digidocs/90001019/default.htm#tasks/t_configure_ethernet_mac_filtering.htm%3FTocPath%3DConfiguring%2520network%2520interfaces%2520%7CConfigure%2520Ethernet%2520interfaces%7C_____6)
"Ethernet MAC filtering restricts which Ethernet devices can send packets to the router. If MAC filtering is enabled on an Ethernet interface, only Ethernet packets with a source MAC address that is configured in the MAC Filter table will be allowed. If the source MAC address is not in the MAC Filter table, the packet will dropped."
I hope this helps, in case you still have issues, please send us an email to firstname.lastname@example.org with IMEI of the device and a debug.txt file (https://www.digi.com/support/knowledge-base/how-to-extract-the-debug-txt-file-from-a-digi-tran) so we can open a case to further assist.
Please also review Digi Support Level options here: https://www.digi.com/support
Digi Technical Support Team