I had this issue also , but one of the bios releases allowed me to use it . That was stopped in the latest bios. I would avoid updating bios at all costs ,others ,including myself ,have found that causes more issues than it cures. Keep an eye on your temps ,cpu and gpu ,and if they are not excessive then leave well alone. If CPU or GPU get overly hot ,then I would suggest perhaps a re-paste of thermal compound ,if still under a warranty get the repair centre to do it. I just had heat issues with a 15-7000 series and ended up with new mobo and fan under warranty. HWinfo or CPU-Z or RealTemp64 should help monitor the temps etc. Others have posted re: undervolting etc. but I am relactant to go there incase of any issues again.

Following the repair the temps are a little better ,but, they still hit 100 degs on a regular basis. I think this something Dell need to sort out by way of bios update or software fan control to give options like Active / Passive /and Performance. I just don't think the fans cut in early enough. If you laptop is less than 30 days old you could always request a replacement.

Dell have released a new bios to fix this exact issue of not being able to change thermal settings.

My laptop is 15 7000 series Model 7580 ,latest bios is 1.7.0 and fixes the issue.

Assuming your updates are the same as my model this bios sorts the problem.

Please check model specific downloads for bios for your model.