iPhone 6 and 6+ have a serious design flaw (in my opinion): the frame is too thin and too soft for the size. It makes the frame and consequently the board inside the frame prone to flexing and bending. This constant flexing and bending is damaging connections between certain chips and the board, as well as some traces.
The two circuits most prone to failure due to this flaw as I've seen them are the touch circuit and the baseband circuit.
Although touch circuit repair at board level is largely successful if all the correct procedures and future proofing are applied (say 90% successful and not coming back), I have yet to see a baseband failure that isn't coming back soon for re-working after failing again (say 10% only successful, which is too low of a yield and I am talking about world-class board level technicians). And this repair is time consuming and not cheap.
So everyone looses when trying this repair: the owner of the device who paid some good money only to see the issue return sometime after - except may be a lucky minority, as well as the technician who put in some serious time only to see the client coming back to repeat the job over and over or to ask for a refund.
In summary, in my opinion, if you iPhone is suffering from "no service" issue using a known good SIM, don't try to repair it. Even if a technician swears they can fix it for you.
If it is as well suffering from greyed Wi-Fi (you cannot turn Wi-Fi on), it is definitely not worth the repair since the Wi-Fi chip will also need to be replaced or re-balled.
If the Wi-Fi is just weak, you could repair that and use it as an iPod. It may be that the antenna came loose or snapped from some drop, may be the same drop that damaged the baseband circuit. And that antenna is different from the one for cellular service. Replacing one will not fix the issue for the second one.