Well, if you believe the stuff about an issue with the motherboard, you would not want to try to charge the phone at all, since that board is involved with both wired and wireless charging among other things.
If the issue is with the battery or the battery is involved.....a possibility....you would not want to continue trying to use the phone.
I think the tech was really trying to say that you need to get the phone serviced at the Apple Store or an authorized Apple service shop as soon as possible.
You can find a list of authorized Apple service shops in your area using this link:
Find Locations
Good luck