In my experience, I got nothing on the RISCV serial port until I found a DDR4 DIMM that worked. You need that first.
Also, even with a DIMM that worked, I have not gotten more than 1 DIMM configuration to work. Only configs that boot are a single DIMM in the slot closest to the PCI slot.
I recommend you confirm you have an Registered ECC DIMM with 1 or 2 Ranks of x8 memory chips. There may be additional requirements (speed, CAS latency (CL), etc…)
There have been at least one report of a x4 memory DIMM working, but my 1Rx4 DIMM didn’t work.