Notifications cannot be received on iOS

The iOS app shows this warning when connecting to my docker based self hosted instance of MM.

Steps to reproduce
mattermost/mattermost-enterprise-edition:8.1.9 with standard ports 8067/tcp,>8065/tcp, :::8065->8065/tcp,>8443/tcp, :::8443->8443/tcp, 8074-8075/tcp,>8443/udp, :::8443->8443/udp
behind an Apache2 reverse proxy which is configured as follows:

<VirtualHost *:80>

        Alias /.well-known/acme-challenge/ /var/www/
        RedirectMatch Permanent "^(/(?!\.well-known/).*)"$1
        ProxyPreserveHost On

        ErrorLog ${APACHE_LOG_DIR}/isagog-error.log
        CustomLog ${APACHE_LOG_DIR}/isagog-access.log combined

behind a firewall that only lets incoming acces to ports 80, 443 and established 32768-65535.

In the iOS client: Settings → Notifications → Push notifications → ALL / Online.away,offline

In the Mac client → Console → Environment → Push notification server → Manually enter → (also tried Use TPNS)

Expected behavior
Receive push notifications

Observed behavior
Little red exclamation mark under the channel name. “Notifications cannot be received from this server”

Push Notification Issue - #3 by zanderisrael may help.