Concur with @Brash. Place a switch behind the ISP to give yourself the additional port capacity to your MXs.
Points to note regarding MX WAN uplink IPs:
use MX uplink IPs: When using this option, the current active MX will use its distinct uplink IP or IPs when sending traffic out to the internet. This option does not require additional public IPs for internet-facing MXs, but also results in more disruptive failover because the source IP of outbound flows will change.
Use virtual uplink IPs: When using this option, both MXs will use a shared virtual IP (VIP) when sending traffic to the internet. This option requires an additional public IP per uplink, but allows for seamless failover because the IP address the network is using to communicate with the internet will be consistent. The VIP for each uplink must be in the same subnet as the IPs of the MXs themselves for that uplink, and the VIP must be different from both MX uplink IPs.
Darren O'Connor | uccert.co.uk
I'm not an employee of Cisco/Meraki. My posts are based on Meraki best practice and what has worked for me in the field.