Router runs VPN then IPFire then MAC

Hello. Version 147. A VPN runs on an openwrt OS Router (ig-net Slate 750) and is always on fed by a Vodafone R219 Hotspot. Followed by IPFire: the VPN works OK but the occasional problem of a lost connection. Does IPFire have a requirement to pass the VPN traffic differently?

related general to OpenVPN another preceding SPI FW can cut an UDP connection under specific circumstances, whereby TCP can help to overcome such problem.

But am not sure if this is the answer which you are looking for since am not really sure of the specifics of your problem.