Clients who block all outgoing udp connections having problems connecting to turnserver

We recently had a problem with our turnserver (coturn), some clients who use firewalls to block all outgoing udp connections have problems connecting to it, as far as i know, when udp connection doesn't work, it should fallback to using tcp right ? but why is this not happening ? we tried opening all traffic in our server to test, but the same issue is still present, how to deal with a situation like this ? from someone who is not very familiar with websockets and turnservers.

We have our turnserver behind a network load balancer (aws) , same issue happen when it is not behind a loadbalaner. This is my config, using ubuntu 16 :

server-name=example.com
cert=/etc/letsencrypt/live/example.com/cert.pem
pkey=/etc/letsencrypt/live/example.com/privkey.pem
realm=example.com
fingerprint
listening-ip=0.0.0.0
external-ip=*.*.*.*/10.0.1.95 #or just the external ip
listening-port=443
min-port=10000
max-port=60000
log-file=/var/log/turnserver.log
verbose

user=user:password
lt-cred-mech