I have a win10 64bit version 1803 and avast free, also on a WiFi connection. I have my program updates set to Manual as I don't want programs (not just avast) to auto update.
I have had no problems in doing a manual program update - Avast Tray Icon > Right Click - Update - Program and that has worked for me on all my systems. See attached images, I did test the update and it did check, but I already have the latest version.
Though it is no bad thing to download the off-line installation file, I save a copy of it if downloaded, in case I ever want/need to revert back to it.