SharePoint 2016 and Office Online Server SSLOffloaded

We have SharePoint 2016 (SP16) and Office Online Server (OOS) behind a local reverse proxy (ARR) and outsourced SSL (SSL certificate installed only on the reverse proxy).

However, when users open files from SP16, they experience two problems:

  1. Warning: "Only secure content is displayed"> must be clicked on "Show all content".
  2. Subsequent requests sent by clients to oos.myexternal.com (proxy) are HTTP and HTTPS in both types, instead of just HTTPS.

Please help with to clarify whether the following details or the additional configuration required for the proxy are faulty. Many thanks.

OOS:
InternalURL: http://oos.myinternal.com/
ExternalURL: https://oos.myexternal.com/
AllowHTTP: True
AllowOutboundHttp: False
SSLOffloaded: True

SP16 bond:
Server name: OOS.myinternal.COM
WopiZone: external-http