R6220 as bridge

> Since it seems like noone is able to answer this in a concrete way,
> [...]

   Huh?  Who was asked what?

> [...] The R6220 is supposed to be able to operate as a bridge.

   Who supposes that?  What, exactly, does "as a bridge" mean to you?

> All instructions I've found [...]

   Where, exactly, did you find what, exactly?

> This is how simple it is to configure a competitor: [...]

   A TP-Link RE200 or RE210 is a wireless extender, not a router (like
an R6220).  Yes, configuring a wireless extender as a wireless extender
might be easier than configuring a router as a wireless extender.
Especially if that router does not support operation as a wireless
extender.

   Netgear makes wireless extenders, too.

> [...] I have to do these steps: [...]

   You omitted the most important step:

   0) Get a device which was intended to do the job you want done.

> [...] I've found answers like: "The R6220 doesn't have that feature"
> [...]

   I'm less amazed by that than you seem to be.