I am presuming you can fire up macOS Disk Utilities in Recovery Mode (cmd-Opt-R) if you can then open up Terminal from the pull down Utilities menu and enter: dscacheutil -flushcache and press return. Reboot back into Recovery Mode and try connecting again "Reinstall macOS" selection. If it asks you to connect to a particular Network select your modem NOT your mac network.

You might find this helpful: How to Fix Mac Recovery Mode not Working on High Sierra?

Cheers and Good Luck