There might be many things, but most frequent is firewall. If You run node via docker image, check that ip_forwarding is enabled on a host and that UDP service port (specified by --openvpn.port flag) is allowed from outside.

check ip_forward status:

# cat /proc/sys/net/ipv4/ip_forward

enable ip_forward if disabled:

# sysctl -w net.ipv4.ip_forward=1

It also might be that default firewall forward policy is set to DROP (check it with sudo iptables-save command). In that case try setting it to ACCEPT. Generic way to do it, provided there are no other interfering rules:

# iptables -P FORWARD ACCEPT

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