I tried installing with:

opkg install https://downloads.openwrt.org/releases/19.07.2/packages/mipsel_24kc/packages/zerotier_1.4.6-1_mipsel_24kc.ipk

but I get the error:

root@GL-MT300N-V2:~# opkg install https://downloads.openwrt.org/releases/19.07.2/packages/mipsel_24kc/packages/zerotier_1.4.6-1_mipsel_24kc.ipk
Collected errors: