As pointed out by @Borromini , do NOT try to literally copy contents of old /etc/config/network file from 19.07 to 21.02. It is incompatible.
You should reset the router to clear old settings and manually set it up from beginning using LuCI.
Once you have configured the dumb AP, try connecting LAN port from Asus openwrt device to your ISP facing router's LAN port.
fwiw, if you are just using the Asus as a dumb AP, have you considered using stock AsusWrt firmware in Access Point mode? AsusWrt offers faster wifi transfer speeds than OpenWrt on RT-AC57u v1 in my experience.