Hi. Hopefully it's a little simpler than you think.
Firstly, the B2 will work with wifi, so there should be no need for an ethernet connection, provided your wifi signal is reasonably good. You can then access the web UI on any desktop, tablet or phone which is also on your wifi network. If necessary, and you have the B2B software, you could add an ethernet cable without opening the back of the B2, using an ethernet to USB adaptor cable and clicking one setting in the B2 (advanced) menu ("get ethernet").
Secondly, regarding B2B: if the B2 in question was manufactured after early 2020, you should not have to change the operating system, so no need to change the SD card. B2B is a better operating system, in the sense that it is more up to date and can run all the latest features. The Bluetooth functionality is better too. The B in B2B doesn't mean Broadband, the older B2 operating system also works on wifi and still gets downloadable updates, but there are a couple of newer features (such as being able to use an ethernet adaptor, as above) which require B2B, so it's better to have it.