I'd go to Crucial.com and see what is offered.
My laptop is the ASUS F556UA-UH71, and its motherboard number is the X556UAK. It has 8GB of RAM soldered on the motherboard itself with the following specs: DDR4 2400MHz, timing: 15-15-15-35. Mind you, even though it's rated at 2400MHz, it runs at 2133MHz due to the processor, which doesn't support frequencies higher than that (i7-7500U). There is an expansion slot and I wanted to expand it to 16GB, not only because of the work that I do and some light gaming, but also to experience the performance benefits of a dual channel setup.
So I did some research to find compatible RAM sticks. If I go with a 2400MHz stick, the best timing that I could find is 16-16-16-39. So my best bet is to go with a 2133MHz, and the best timing on those are 15-15-15-36. Actually, there aren't too many options available for me, the only one that I can choose is the G.SKILL Ripjaws F4-2133C15S-8GRS
My question is, will I run into problems by installing this stick, due the different, but similar timing? Will the difference in frequencies cause problems, even though the 7500U doesn't support higher frequencies than 2133MHz and the one on the motherboard is rated at 2400MHz?