This occurred because BIOS automatically lowered memory frequency to allow inter-module compatibility.
Like most laptops, the BIOS is severely limited in available user adjustments. This usually means memory and voltage adjustments are not available to the user.
There appear to be two options -
1) Try another 2666 MHz memory module and see if that will work together with the existing module at 2666 MHz. But be prepared to try several modules until a fully compatible combination is found. This process may take some time and may cost more in the end.
2) Get a system compatible dual channel kit with two compatible modules in it at 2666 MHz to replace the existing modules. The kit modules will have been tested for compatibility and are guaranteed to work together. This might be the easiest, quickest and cheapest option(?) The existing modules could be kept as backup, or sold.