The following ports have to be opened in your firewall:

3478/tcp+udp (TURN)
3479/tcp+udp (TURN)
5349/tcp+udp (TURNS)
5350/tcp+udp (TURNS)
49152:65535/tcp+udp (Relay ports)

If you use ufw, this can be done with a single command:

sudo ufw allow turnserver
sudo ufw reload

First, generate an SSL certificate for your subdomain, and create a post-renewal hook for certbot.
Coturn does not run as root, so it cannot read from /etc/letsencrypt/live/.

sudo certbot certonly -d turn.quietlife.nl

First create a directory readable by Coturn:

sudo mkdir -p /etc/coturn/certs
sudo chown -R turnserver:turnserver /etc/coturn

Then create the script certbot should run after renewing the certificate:

/etc/letsencrypt/renewal-hooks/post/coturn.sh

#!/bin/sh
cp /etc/letsencrypt/live/turn.quietlife.nl/fullchain.pem /etc/coturn/certs/
cp /etc/letsencrypt/live/turn.quietlife.nl/privkey.pem /etc/coturn/certs/
chown turnserver:turnserver /etc/coturn/certs/*
chmod 600 /etc/coturn/certs/*
systemctl restart coturn.service

Then make the script executable:

sudo chmod +x /etc/letsencrypt/renewal-hooks/post/coturn.sh

Finally run it manually once:

sudo sh /etc/letsencrypt/renewal-hooks/post/coturn.sh

sudo apt install coturn

You will have to set a number of options in /etc/turnserver.conf. There are a lot of commented examples in the default file, so these are only the lines you have to add or change:

/etc/turnserver.conf

listening-ip=93.119.10.240
listening-ip=2a01:7c8:bb09:3dc::1
 
use-auth-secret
static-auth-secret=dddddddddddddddddddddddddddddddddddddddddddddddddd
realm=turn.quietlife.nl
 
user-quota=12
total-quota=1200
 
no-tcp-relay
 
cert=/etc/coturn/certs/fullchain.pem
pkey=/etc/coturn/certs/privkey.pem
 
syslog
 
no-multicast-peers
 
denied-peer-ip=10.0.0.0-10.255.255.255
denied-peer-ip=192.168.0.0-192.168.255.255
denied-peer-ip=172.16.0.0-172.31.255.255
denied-peer-ip=0.0.0.0-0.255.255.255
denied-peer-ip=100.64.0.0-100.127.255.255
denied-peer-ip=127.0.0.0-127.255.255.255
denied-peer-ip=169.254.0.0-169.254.255.255
denied-peer-ip=192.0.0.0-192.0.0.255
denied-peer-ip=192.0.2.0-192.0.2.255
denied-peer-ip=192.88.99.0-192.88.99.255
denied-peer-ip=198.18.0.0-198.19.255.255
denied-peer-ip=198.51.100.0-198.51.100.255
denied-peer-ip=203.0.113.0-203.0.113.255
denied-peer-ip=240.0.0.0-255.255.255.255
denied-peer-ip=::1
denied-peer-ip=64:ff9b::-64:ff9b::ffff:ffff
denied-peer-ip=::ffff:0.0.0.0-::ffff:255.255.255.255
denied-peer-ip=100::-100::ffff:ffff:ffff:ffff
denied-peer-ip=2001::-2001:1ff:ffff:ffff:ffff:ffff:ffff:ffff
denied-peer-ip=2002::-2002:ffff:ffff:ffff:ffff:ffff:ffff:ffff
denied-peer-ip=fc00::-fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
denied-peer-ip=fe80::-febf:ffff:ffff:ffff:ffff:ffff:ffff:ffff
 
allowed-peer-ip=93.119.10.240
allowed-peer-ip=2a01:7c8:bb09:3dc::1
 
no-cli
 
no-rfc5780
no-stun-backward-compatibility
response-origin-only-with-rfc5780

Make sure that you change listening-ip and allowed-peer-ip to the external IP address(es) of your server, and that you set realm to the subdomain you want to use. For static-auth-secret, generate a random string.

Then restart Coturn:

sudo systemctl restart coturn.service