For all who read this:
There is incorrect information circulating in the forums . The maximum addressable memory on an Intel J3355 or J3455 CPU is 8 GB per the Intel "Ark".
The Synology BIOS reads the DIMM information, totals the value, and displays to you what it finds. It cannot use more than 8 GB.
For best use in Plex, if you are adding memory, you should upgrade using an '8 GB kit' . These come as two matched sticks of 4 GB. This is what allows dual channel mode to enable and speed up hardware transcoding by doubling total memory bandwidth available (2 paths of 64 bits instead of 1 when mismatched). It will not improve CPU audio transcoding performance but does allow the GPU/ASIC to better coexist with the CPU, which is doing audio transcoding, resulting in less contention for the memory bus when both are occurring simultaneously.
Similarly, the J3355 CPU also has the same 8 GB memory limitation.