The TL-SG105E supports one group of static LINK Aggregation, and it is pretty easy to do it, put the ports you want to aggregate to the group and then connect the ports. But you need to mind two things, first, both side need to support Static LAG/ Port Trunk; second, the port that you want to put into the group should have the same properties, if you are not sure about that, please do the LAG before you configure other settings.

Thank you for your prompt reply.
"Put the parts you want to aggregate to the group and then connect the ports" - that's my problem - Where?

KR
Hi-Fey

you can refer to the UG page 18 for instruction.
http://www.tp-link.com/res/down/doc/TL-SG105E_V2_UG.pdf

Also thank you for your answer.
After I "assembling" the two trunk ports does my Synology NAS continue "IEEE 802.3ad link could not be established."