In OCS R2 Desktop Sharing Uses RTP/(RDP For Web Access Users) as AV. In OCS R2 you dont need any Public IP for AV to work from Internet. Check if you have 3478 UDP and 443 TCP(Default) opened from Internet to Edge server. I have seen most of A/V issues happens because of bad routing between your AV Edge and Internal network or with ports. Please look in to the followinf Diagram for OCS R2 port requirement. Hope this Helps.
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Jithendranath Reddy