ISP(Internet Service Provider) provides Internet services to businesses and personal users.
In contrast, carriers maintain the Internet services.
One carrier organization often consists of multiple ISPs, yet one ISP only belongs to one carrier.
You can choose either one to target under "Campaign Management"->"Targeting". However, we would more suggested you target by ISP because it's more specific and precise.
You can check the dimensional report to see all the impressions, clicks and conversions coming from which ISPs.