Binding SP2013 farm to Office Web Apps server - "The server did not respond"

Hi,  I have an SP2013 farm I'm trying to connect to an internal only OWA server.  I have a load balancer to offload SSL to and have been following this doc ff431687#SPHTTPS, but keep getting "Warning: The server did not respond. Trying again (attempt 1 of 5)."  when using the command "New-SPWOPIBinding -ServerName "servername.mydomain.com""

Firewalls on SP Farm and OWA box are disabled.  I can telnet on 80 and 443 from the CA/WFE to OWA OK.

I've already set up the OWA server using:

"New-OfficeWebAppsFarm -InternalUrl "https://servername.mydomain.com" -SSLOffloaded -EditingEnabled"

"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe -iru"

The "Get-OfficeWebAppsHost" command returns my domain in the allowList.

Any suggestions would be gratefully received.

Thanks.