I need to connect my SL-55 to and external antenna outside the
building, however this phone doesn't have a connector, so I can't
attach it directly

Do you know where to buy (or how to make) some antenna coupler that I
could use?

I know of an "antenna patch" that sticks in the back of the phone.
That would be a solution, but it's sold in the United States and
shipping costs to Europe are too high for me.

I would really appreciate any information you could provide me!

Thanks. Gabriel