@Paolo-Moro well, i just uncomment all the repositories and have had good luck in finding with opkg but
openwrt does have a web interface for the packages.
determine what version of firmware you have.
you can then start here,
https://downloads.openwrt.org/releases/
but if 17.01 then here,
https://downloads.openwrt.org/releases/packages-17.01/mipsel_24kc/base/
if 18.06 then here,
https://downloads.openwrt.org/releases/packages-18.06/mipsel_24kc/base/

for future reference you might want to bookmark these,
https://openwrt.org/packages/pkgdata_lede17_1/ipset
https://openwrt.org/docs/guide-user/firewall/fw3_configurations/fw3_config_ipset
https://openwrt.org/packages/table/start?dataflt[Name_pkg-dependencies*~]=ipset
https://openwrt.org/packages/start

and as always google search is usually helpful.