Interrupted calls (turnserver)

setup:
ubuntu 18.04 lts
php 7.2
coturn
server is behind router (NAT)

Calling working (video and audio), but they are interrupted after some seconds / minutes of call time.
Participiants are behind routers / nat, so they are using turn server.
Can you help me?
my coturn conf:

listening-port=3478
tls-listening-port=5349
external-ip=17x.xxx.xxx.xxx
verbose
fingerprint
lt-cred-mech
use-auth-secret
static-auth-secret=32xxxx
server-name=xxxx
realm=xxxxxxx
realm=xxxxxxx
cert=/
pkey=/
cipher-list=
dh-file=
log-file=/
no-tlsv1
no-tlsv1_1

Im not using turn-tls port. I tried many things and calls are interrupted.