Hi
Actually it should be possible…
But I never setup a server with DHCP! (Not even with reserved DHCP IPs! I prefer a fixed IP, and if the configuration is somehow lost, then it uses DHCP with the same IP address. I use this setup a lot for network printers.)
I always use fixed IPs for Servers, weather they are running for real or virtualized, even if the only reason is one failure zone less.
(The servers will work and be accessible even if DHCP is down.)
I used internal IPs for the VMs, but forwarded Ports or 1:1 NAT for the VMs needing external access.
In the case of the Hetzner Proxmox, the OPNsense firewall (Also free as NethServer) did have a small DHCP pool (10.0.0.241-254). This was to keep the DHCP pool small, but the range of usable IPs big, for fixed or reserved IP installations.
My 2 cents
Andy