i want to upgrade laptop ram with compatible timing (CL - cas)

Question

sorry my english not very good

this is my laptop (Acer aspire 5 A515-51G-5400)

i want upgrade my laptop ram to 12gb with 8gb but i confused about what timing "CL" should buy

i have now 6gb in my laptop 4gb + 2gb i want remove 2gb and add 8gb

the specifications of both rams are ddr4 2400Mhz - 1.2v - CL17 but the CPU-Z show me different specifications

speed is 2128 or 2133 not 2400 and CL is 15 not 17

these pictures of my ram and cpu-z and ramexpert

https://i.imgur.com/BrgBdmU.jpg

https://i.imgur.com/gxOSTww.jpg

https://i.imgur.com/k2b6ylT.jpg

i know my cpu (i5-7200u) doesn't support more than 2133 that why my ram is 2133 - CL15 not 2400 - CL17 because i think when speed decrease also CL decrease (but not very sure)

but now what specifications of ram should i buy

1- 8gb ddr4 2400 1.2v - CL15 (because timing of my ram now is 15 but i worry than CL15 will decrease to 13 because the speed will decrease to 2133)

2- 8gb ddr4 2400 - 1.2v - CL17 (i think it will work because the specifications like my 2gb ram but maybe there is better choice for my laptop)

3- 8gb ddr4 2133 - 1.2v - CL15 (maybe it is best choice but i don't know if it will work fine or not because my specifications of my main ram is 2400 - CL17 not 2133 - CL15)

which one should i buy

and please tell me if there is better than these choices

thank you