Can i install RAM of higher frequency into Latitude 5590?

I have an 6 years old laptop Dell Latitude 5590. Recently i realized its 8 GBs of RAM are no longer enough so i would like to upgrade. The manual says the original memory modules operate on 2400 MHz, but memory sticks with such frequency are no longer available anywhere in my country, i can only buy ones with a higher frequency.

So my question is: Can i replace the current memory module with a new one of higher frequency and capacity? Will the motherboard be able to downclock the memory to its supported frequency or will it just refuse to work?

And optional complementary question: Is it better to install 1 or 2 memory sticks. The current setup has 1.