This needs extra planning, since network isn't avaialable without it. Download the needed packages (see below) manually, before upgrading, from:
- Kernel-land: https://downloads.openwrt.org/releases/18.06.2/targets/ar71xx/generic/
- User-land: https://downloads.openwrt.org/releases/18.06.2/packages/mips_24kc/base/
After upgrade, copy them to the router: