It worked after including
{
servers {
protocol {
allow_h2c
}
}
}
as mentioned in Global options (Caddyfile) — Caddy Documentation
Complete Caddyfile:
{
servers {
protocol {
allow_h2c
}
}
}
:80 {
route /host.Service/* {
reverse_proxy h2c://server:5300
}
}