If your router can set up the port speed, you can try to set up 1000M as the port speed. Or try to change the cable to chekc whether the issue is related to the cable.

I check the tpNMS on the official website of TP-Link. But I find that it shows 'end of life' . Maybe you can find other SNMP softerware.

I have TL-SG2008 and I like TP-Link too. I try to reboot the switch but the speed is still 1000M. But what I connect to the switch is PC, maybe different with Fritzbox. As I know, if we set up the...

Do you connect the PC to the switch directly? And you can try to remove other devices temporarily for the test.

As I know, T2600G-28MPS should support multivlan routing. Do you mean that the switch has three VLAN and you have set up the IP address for these VLAN? Maybe you can set up the static routing on the...

Maybe you can test like PC---switch(SFP)----(SFP)switch---PC, use two PCs to ping each other. Maybe it's not a best way. If you have enough money, you can buy the new.