I've spent hours researching this and have only found one solution. You have to buy an ATT go phone prepaid SIM card kit for $2 online and install it. Get a "Pay by the minute" plan (it won't let you checkout without selecting the plan, but you don't have to use it).
Once you get the SIM, don't activate it. You can simply install it and then check for updates over wifi. The G6 makes you wait 24 hours between each time you check for updates. You can either just wait until the next day or set system clock to a later date manually.
I've done this twice now to keep it updated. It's embarrassing that in 2018 LG can't push their own OTA updates.