I recently bought a used CRS 326-24S+2Q+RM. I connected it to an intel x520 da2(bought used and installed) installed in my server using an OM3 cable. I am encountering this issue where when SFP+ optics are set to auto negotiate or set to 10g the upload speed is stuck at 20 Mbps (500 mbps fiber optic internet line). When I set the speed in to 1g in the routeros this problem disappears. Since iperf is not avaiable I am using speedtest and iperf from from my pc with gigabit Ethernet to check the performance. I have tried using a different om3 cable (both of them are brand new) and using windows and linux with the x520.Additionally I have also tried it using both intel and finisar sfp+ modules, but the issue still persists. Though I have noticed that this problem does not occur when I tested using sfp+ ethernet modules from 10gtek. Any pointers or help in this regard would be great. Though I have noticed that this problem does not occur when I tested using sfp+ ethernet modules from 10gtek.

If you have a Mikrotik router on either end you could use the Mikrotik traffic generator, got a stress test guide below if interested
https://shop.duxtel.com.au/help/en-gb/5/25

I installed router os on my hypervisor virtually and did the speed tests. They were looking ok. I installed a connectx-4 and ran iperf from a my terminal workstation to my server and I got 5gbps. It seems that the switch is ok but again running ookla speed test, the upload speed is greatly affected. I have tried using flow control, but I get into the same issue.
I have identified that the switch has some trouble communicating with bell home hub router.
When I put a pfsense box with intel i350 in between the switch and the ISP bell home hub router the speed increases to 250 Mbps but there is a lot jitter when using remote desktop.