I thought I had the answer with this as well but this is not working for me. I have my Dynamic DNS hostname in place which I use for other services so I know it's resolving properly. However, when I try to connect from my phone I see it trying to connect to the Docker Containers IP address which is 172.12.0.5. I should mention that the Open Port Checker at https://www.yougetsignal.com/tools/open-ports/ shows that 9443 is open. So I know it's got to be an issue with the app config. Here are a few screen grabs of my network config for OpenVPN-AS. I temporarily masked my Dynamic DNS address for the grab.
Here is the log file from the OpenVPN client app on my iPhone where I have the Dynamic DNS address set as the hostname and port 9443 as the port:
2019-07-09 07:07:47 ----- OpenVPN Start -----
OpenVPN core 3.2 ios arm64 64-bit PT_PROXY built on Oct 3 2018 06:35:04
2019-07-09 07:07:47 Frame=512/2048/512 mssfix-ctrl=1250
2019-07-09 07:07:47 UNUSED OPTIONS
11 [sndbuf] [0]
12 [rcvbuf] [0]
15 [verb] [3]
24 [CLI_PREF_ALLOW_WEB_IMPORT] [True]
25 [CLI_PREF_BASIC_CLIENT] [False]
26 [CLI_PREF_ENABLE_CONNECT] [True]
27 [CLI_PREF_ENABLE_XD_PROXY] [True]
28 [WSHOST] [172.17.0.5:9443]
29 [WEB_CA_BUNDLE] [-----BEGIN CERTIFICATE----- [Certificate Masked]...]
30 [IS_OPENVPN_WEB_CA] [1]
31 [ORGANIZATION] [OpenVPN Inc]
2019-07-09 07:07:47 EVENT: RESOLVE
2019-07-09 07:07:47 Contacting [172.17.0.5]:9443/TCP via TCP
2019-07-09 07:07:47 EVENT: WAIT
2019-07-09 07:07:58 Server poll timeout, trying next remote entry...
2019-07-09 07:07:58 EVENT: RECONNECTING
2019-07-09 07:07:58 EVENT: RESOLVE
2019-07-09 07:07:58 Contacting [172.17.0.5]:9443/TCP via TCP
2019-07-09 07:07:58 EVENT: WAIT
2019-08-09 07:08:09 Server poll timeout, trying next remote entry...
2019-08-09 07:08:09 EVENT: RECONNECTING
2019-08-09 07:08:09 EVENT: RESOLVE
2019-08-09 07:08:09 Contacting [172.17.0.5]:9443/TCP via TCP
2019-08-09 07:08:09 EVENT: WAIT
2019-08-09 07:08:19 EVENT: CONNECTION_TIMEOUT [ERR]
2019-08-09 07:08:19 Raw stats on disconnect:
CONNECTION_TIMEOUT : 1
N_RECONNECT : 2
2019-08-09 07:08:19 Performance stats on disconnect:
CPU usage (microseconds): 49938
Network bytes per CPU second: 0
Tunnel bytes per CPU second: 0
2019-08-09 07:08:19 EVENT: DISCONNECTED
2019-08-09 07:08:19 Raw stats on disconnect:
CONNECTION_TIMEOUT : 1
N_RECONNECT : 2
2019-08-09 07:08:19 Performance stats on disconnect:
CPU usage (microseconds): 51440
Network bytes per CPU second: 0
Tunnel bytes per CPU second: 0
Any assistance is greatly appreciated.