The ipk on 17.x is compatible with 18.06. It should works as follow:

root@GL-AR300M:~# wget http://downloads.openwrt.org/releases/18.06.0-rc1/packages/mips_24kc/packages/fping_4.0-2_mips_24kc.ipk -O /tmp/fping_4.0-2_mips_24kc.ipk
Downloading 'http://downloads.openwrt.org/releases/18.06.0-rc1/packages/mips_24kc/packages/fping_4.0-2_mips_24kc.ipk'
Connecting to 148.251.78.235:80
Writing to '/tmp/fping_4.0-2_mips_24kc.ipk'
/tmp/fping_4.0-2_mip 100% |*******************************| 15254   0:00:00 ETA
Download completed (15254 bytes)
root@GL-AR300M:~# opkg install /tmp/fping_4.0-2_mips_24kc.ipk 
Installing fping (4.0-2) to root...
Configuring fping.
root@GL-AR300M:~# fping -h
Usage: fping [options] [targets...]

Probing options:
   -4, --ipv4         only ping IPv4 addresses
   ......