Rest-API access outside firewall in order to allow integration

hi All,

Our instance is behind firewall and currently only accessible via VPN and not accessible over internet.

we have implemented air watch solution for our field staff to access Sugar on the go. We would like to have some solution for our external partners to send us data via API (leads / prospects ) also collect customer responses (Meeting acceptance link [which is currently not working] , pools and surveys) to be recorded back in CRM behind firewall.

whats the best way and less code intrusive / customisation way of acheving this.

thanks in advance

Kunal