How to allow Envoy process HTTP/2 over cleartext?

We have a service architecture hidden behind an Envoy proxy, that is in turn hidden behind a Nginx acting as a proxy and a TLS termination handler, therefore all internal communications are passed without any encryption. Some time ago the proxy has been updated to handle HTTP/2 over TLS due to external request, which made Nginx forward request to our Envoy with HTTP/2, which it rejected, then Nginx tried forwarding with HTTP/1.0 and Envoy rejected again with HTTP error 426. We have made a work-around by instructing Nginx to forward requests with HTTP/1.1 protocol, yet we still want to make Envoy handle and process HTTP/2 streams over clear text. The problem is, the HTTP/2 support is declared for our Envoy version (1.14.1, we can update to 1.19 if needed), the HTTP processor seems to be separated from TLS handling, yet the manual on Envoy only has instructions on how to use HTTP/2 over TLS without any single hint on how to enable HTTP/2 over cleartext processing. What do we need to allow Envoy process HTTP/2 traffic over clear text?