Why can openVPN not work when changing the firmware to openWRT?

Question

openvpn

There is an ASUS RT-AC57U router that recently had openwrt firmware installed, to be more precise

openwrt-19.07 branch (git-20.247.75781-0d0ab01) / OpenWrt 19.07.4 r11208-ce6496d796

After configuring the WAN and other parameters, everything works fine except for the external VPN. (When using the original Asus firmware, everything worked fine). The OS from which the connection is made is arch linux

The VPN connection itself is successful, which is recorded in the logs on the openVPN server itself, and the corresponding lock icon lights up in networkmanager, while listening to packets via tcpdump on the router itself, packets are visible on the remote openvpn port.

UDP port of openVPN is not standard (not udp/1194 but udp/51194). The symptoms are as follows, after turning on the VPN - neither ping nor nslookup works, nothing actually works, ssh to other servers also does not work. It seems that the traffic is blocked by the firewall on the router, but there are no settings related to this, by default it should be open, which I was kindly informed on the openwrt forum.

Question. How to debug this problem? Perhaps someone has already experienced this. I emphasize that the openVPN server is not running on a router, but on an external remote host.

At the same time, the tun0 interface in arch-linux receives an ip address from the local network 192.168 - from the same one that the router distributes to connected devices via DHCP, is this normal?

Firewall settings in openWRT are default, two zones WAN, LAN and some default traffic filtering rules.