VPN Tunnel under NAT

Hello,

I have 2 MIkrotik routers

  • first have a Public IP Address configured on its WAN
  • 2nd is under 3rd party NAT

I need to estabilish a VPN tunnel over these two routers, but I don’t have access to the 3rd party firewall

What’s the best type of tunnel purpose?

TIA
Stefano