Can The ubuntu server ping the Meraki? Does the ubuntu server have a default Gateway that is pointing to the Meraki? What External IP is the ubuntu server leaving on the internet, is it showing the IP that you set in the 1:1 NAT? Do you have both a Forwarding Rule and a 1:1 NAT rule pointing to the same device, If so try removing the forwarding rule and just keep the 1:1. Any Layer 7 Firewall rules blocking the connection? Any IPS logs show blocks on SSH? Did you confirm this second public IP is working?
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