Hello, good news - the person beeing able to micro solder my SIM slot was finally back from vacation and it worked!
Unfortunately, it is not the easiest thing to do and you have to have some experience in soldering and use a binocular or similar to actually see what you are doing.
One pictures shows the removed SIM slot. As you can see, there are 6 contacts on the upper part of it - but when the slot is attached, you can only see 5. That is a problem, because you can not solder behind the metal cover. So we
bended it a little upwards and back after soldering it, worked. Also, the two slots are connected to each other, so you have to to that as well. it is important to attach the metall part in a proper way so it pushes down the SIM card to make contact.
The second pic shows the soldered in SIM-slot.Unfortunately I forgot to take a pic before putting it back on the silicon part, and I dont have the tools at the moment to take it apart again, but maybe you can see enough here. You can see the soldering between the two Slots - this is done twice, on the upper part and further down. Also you can see some soldering between the contacts.
All together it took approximately an hour (taking apart the phone, unsolder the old slot, solder the new one and put everything back together). So take your time when you plan on doing this.
I hope this information helps.