vpn n2n service degradation when client changes its dynamic ip.

I apologize for my poor english
I have two ipfire firewalls that maintain an active n2n openvpn. The ping between them is 6 ms. However, the client has a dynamic pppoe ip so, although the server points to a dyndns, when the ip change occurs the ping degrades between 200 to 800 ms. The n2n server cannot be restarted so I restart the n2n manually. The problem is not solved. The only way to get ping back to normal is by changing the port on the n2n. Any suggestion?