Question Ram frequencies

A perennial question.
Here is my stock answer:

Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.

If you do buy more disparate sticks, they should be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
I might guess 90% success for intel and less for amd.

What is your plan "B" if the new stick/s do not work?

If you want 16gb, my suggestion is to buy a 2 x 8gb kit.
Yes, it will work in dual channel mode.
Sell the old stick or keep it as a spare.