i assume that the uci config interface for openvpn does not support your cipher.
Please try using a original openvpn config file like in my example below.
config openvpn custom_config
# Set to 1 to enable this instance:
option enabled 1
# Include OpenVPN configuration
option config /etc/openvpn/my-vpn.conf
Here is the package definition of the openvpn package in the openwrt source for 19.07:
If you wanna build a updated version of openvpn for 19.07, then you have to:
- Download the SDK for 19.07
package/network/services/openvpn/Makefile to your needs
- Probably fix some things with QUILT
- Build your Package
- Install the new ipk package on your router
Take a look here how to use the SDK: https://openwrt.org/docs/guide-developer/using_the_sdk