Rubrik support Tunnel access not working

I am need of some help, we have a Fortigate 401f that is being utilized for internet off load. 

RFC19818 to NON-RFC1918 go out the local internet pipe. 

Rubrik support tunnel requires https/443 access to establish a connection to their proxy.rubrik.com site. 

I have created a firewall rule to all my Rubrik Nodes access to ANY with ANY services allowed with AV, APP, IPS and SSL inspection enabled. 

We have asymroute enabled. 

On the pcaps, I am seeing a lot of retransmits and I am wondering if this is a possible mtu or mss size issue, but I am not sure how to find that or determine that information from the pcaps. 

I think this issue is also causing the same problem with another app that keeps timing out and on it I see retransmissions and a lot of client-rst. 

I would I go about detemining the correct mtu/mss size for these applications and I would I make these changes per the policy. 

Thanks,