Alta Labs Route 10 SFP+ ports detect FS.com and 10Gtek DACs but never link; SR optics work at 10G

On an Alta Labs Route 10 the SFP+ ports recognised FS.com SFP-H10GB-CU2M and 10Gtek CAB-10GSFP-P2M DACs (which had worked on MikroTik CRS devices) but no link came up against a MikroTik CRS309-1G-8S+; forcing 1G baseT full gave a 1 Gbps link only. Staff requested diagnostics and ethtool -I output, which showed the router advertising hard-coded link modes instead of the transceiver's capabilities; editing /cfg/sfpX.txt to 10gbae_r or 10gbase_cr did not help.

• Hardware: Alta Labs Route 10, MikroTik CRS309-1G-8S+
• Transceivers: FS.com SFP-H10GB-CU2M DAC, 10Gtek CAB-10GSFP-P2M DAC, FS.com SFP-10GSR-85
• Form factor: SFP+, DAC

Symptoms / messages:

DAC detected but no link
only 1G with forced baseT
hard-coded advertised link modes