Microsoft Surface USB-C to Ethernet (please open related Librem 5 Community Wiki link) … or other similar will work with L14 too (or some future version of L15/L16), as I described (sort of) RTL8153 chip here or officially confirmed as Realtek RTL8153 based USB3.0 Ethernet adapters here.
I hope that this approach helps (to understand at least). It is just one working (take this, please, as direct approach and not some kind of workaround) sample showing that there are different approaches and different ways to get Librem products work out-of-the-box with several ready-made Ethernet adapters to be connected to the particular USB Type C port (using PureOS as default OS). In short, my main advice here would be to (start to) consider USB Type C approach when … without expecting (as/if not happening) that Purism provides some/any/clear promise/solution to your needs (Did I pass?).