Just ssh to router and backup the config by using nvram show
Relevant part is dhcp_staticlist=<00:11:00:00:11:16>192.168.1.16><00:11:00:00:11:17>192.168.1.17>
I know > is confusing, but it has some logic easy to follow.
Router does restart once you click on "Enable Port Forwarding" and you may need to create a static entry prior you manually restoring all entries.
As L&LD is saying, restoring full config from a previous release kinda defeats the purpose of a factory reset. You can restore only small pieces of config, only ones you're sure it does't get "too deep".
At least that's the way I'm restoring DHCP reservations and the similar for port forwarding. It never changed in many years of using Asus devices, no matter the firmware.
nvram commit at the end.