Huh. That's one item I've not seen available for a cellphone online.
Though I don't remember my antennas being attached in such a way as to be at risk of this kind of damage. Got pictures? Are you sure you're not talking about the mainboard-to-USB-charging-subboard cable? I can think of a few ways you might have torn that. Those are available on eBay:https://www.ebay.com/itm/Original-High-quality-Mobile-phone-accessories-For-Blackview-bv9500-BV9500-pro-/132928550514?nav=SEARCH
If I'm imagining what happened correctly, it may be a simple fix. I assume here that you're taking about the actual antenna and not the cable running to the antenna or one of the subboard cables. If so, it's just flat copper. Scrape/sand off any coating from the pieces about 2 to 4 millimeters from the tear. Tape the torn-off piece to the back cover with it touching the other piece edge-to-edge but keeping the sanded metal section uncovered. Then you have two options:
1. Apply a thin layer of solder to the scraped/sanded surfaces using a properly heated soldering iron, using the solder as a sort of bridge. Try to keep it thin. Once cooled, apply some thin tape to prevent contact with other circuits inside the case.
2. If you don't have a soldering iron and don't want to purchase even a cheap one, take a small piece of aluminum foil, and place it across the sanded antenna metal. Apply thin tape. The foil will bridge the gap between the antenna fragments, making it electrically one single piece. Ghetto fix, this method, but it will work.
I suspect though that you are actually talking about the USB sub board cable. Don't try the above methods with that cable. Just replace it.