All right. So not to worry as it does not affect this setup.
Maybe I go a bit off topic here, but after I finalised the setup I did a couple of speed tests (via dslreports) and I get about 12 Mb lower actual download speed (not sync speed) compared to the download speed (~60Mb) when the devices work individually as all-in-one gateways using their internal modems. Does this preformance drop relate to the famous Bt hub 5a wan-lan speed issue or is it something more or less expected?