That is going to be tough. The 997c SI Combo can only be linked to other units with the AS INTERLINK accessory. That accessory is discontinued. It does GPS data sharing only.
IF you can find one, no model made in the past 4 years will work with that connection.
Discontinued models that will are the ones ending in 8 like the Side Imaging units 998c SI Combo and 998c HD SI Combo. Both of those are newer than the 997.
When the XX9 models came out, they were only connectable with Ethernet as well as the current Helix models in the 9, 10, and 12 series.
The XX8 models are INTERLINK and Ethernet compatible.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1