Failed to relay with UDP, while TCP succeeded.

Hello All,

I am trying to deploy a turnserver for my WebRTC application, but connection test failed with UDP.

My config file is shown below:

```
external-ip=my.public.ip.address
channel-lifetime=7200
lt-cred-mech

user=MyUserNamer:MyPassWord

log-file=/var/tmp/turn.log

listening-port=8007

realm=www.mycompany.com
```

The listening port 8007 is opened for both TCP and UDP. It can be verified via commands below:
```bash
# verify TCP connectivity
$ telnet my.public.ip.address 8007

# verify UDP connectivity
$ nc -vuz my.public.ip.address 8007
$ Connection to my.public.ip.address 8007 port [udp/*] succeeded!
```

When I tried to test relay with https://webrtc.github.io/samples/src/content/peerconnection/trickle-ice/, TCP succedded while UDP failed.

The URL I used for TCP is `turn:my.public.ip.address:8007?transport=tcp`, while the ones used for UDP is `turn:my.public.ip.address:8007?transport=udp` or `turn:my.public.ip.address:8007`

I got response for both URLs of UDP:
```
The server stun:103.177.28.44:8007 returned an error with code=701:
STUN server address is incompatible.
```

Can anyone give me some advice, what might be wrong with my environment or usages?.

Many thanks!