Great, thanks for the updates. So just to confirm, AutoVPN S2S tunnels won't come up if it's 3rd party VPN, only when it's MX to MX. Sorry if I'm stating the obvious there, just wanted to clarify if you maybe had a combo of S2S tunnels, some configured via 3rd party and some configured with AutoVPN to other MXs, and maybe the AutoVPN tunnels were not coming up?
You also mentioned a 3rd party FW in the path, so if it's MX to MX and your AutoVPN isn't coming up, make sure the 3rd party FW is allowing UDP/9350 for VPN Registry communications, as well as outbound higher-numbered UDP ports for the UDP hole punching for tunnel establishment. If it's the auto-NAT traversal that's not working, more info here https://documentation.meraki.com/MX/Site-to-site_VPN/Automatic_NAT_Traversal_for_IPsec_Tunneling_bet...