Synology NAS devices run a customized Linux distro called DSM. When executing wg-access-server docker container on DSM I get this error:

time="2021-02-07T12:09:39Z" level=fatal msg="failed to set ip tables rule: running [/sbin/iptables -t filter -C WG_ACCESS_SERVER_FORWARD -s 10.44.0.0/24 -j REJECT --wait]: exit status 2: iptables v1.8.3 (legacy): Couldn't load target ``REJECT':No such file or directory\n\nTry iptables -h' or 'iptables --help' for more information.\n" file="main.go:102"

This is caused by DSM iptables lacking the REJECT target kernel module.