Modem/Router has a weird connector instead of RJ45

Wouldn’t I still need to buy some sort of adapter (usb3 to rj45) if I wanted to test the rPi4 in bridge mode? The rPi4 does only have 1 rj45 port so I wouldn’t be able to test it unless I was just using wireless.

Yes, you are correct, to use more then 500/500 you need a USB3 ethernet adapter (and to use 500/500 with only the pi's built-in ethernet you would need an additional managed switch). Otherwise the pi can not act as router.
I am not trying to guide you away from your plans, by the way, I am quite curious about the USB SFP adapters myself, but I am not sure whether these will work with OpenWrt, while the RPI4B + UE300 combination seems to be tested already be a number of users. So this seems a safer starting point.

That cheapo Chinese USB to SFP adapter has the exact same Realtek chipset as the UE300.

I am not against any of you* trying that, all I want is to inject a word of caution that it might not work. But I am always eager to learn, so if it works even better :wink: That said, if it contains a Realtek chipset will it act as bridge between the SFP module and an USB ethernet adapter and simply present as ethernet adapter to the USB stack?

*) I am behind a 100/40 VDSL2 link, so I will not be able to meaningfully participate in the fun. Sure there are SFP VDSL modems, and I have a router with an SFP cage, but honestly that sounds like a waste of time and not at a meaningful to this thread.

I don't think you need SFP+ here. It looks like the GW05LD router's port is not 10 Gb. Consider a managed switch with SFP ports like the GS1900-10HP. Granted that costs about $150 but it does other things that you'll likely find useful for your network. You could start out with router on a stick or with a USB to Ethernet adapter dedicate one of the Ethernet ports to a VLAN that only converts SFP to Ethernet.

Is it possible for this ISP to provide you a regular wall-mounted ONT like everyone else uses, instead of this unusual setup?

From what little I can see (no specs easily available), it appears the Free modem itself has an SFP+ port on it even if their router is only SFP.

This is an ISP that knows its stuff and is consistently out with market leading products. I wouldn't be surprised if within the next year or so, they have a service that is faster than 1Gbe

What you need is called an sfp transceiver, and it converts sfp to rj45 gigabit ethernet. Something like this:

Then you simply proceed as usual

Note the pictures you give are terrible so I'm going on the idea that the sfp device is female and you'd plug the transceiver into this female sfp if the sfp is male... Forget this and buy yourself a good switch tp-link T1600g-28ts and plug into that

If that's a male sfp another cheap option is

Yes, I agree but then it will be rather a question of how that strange SFP thingy provided by that ISP will behave. You could simply plug-in a regular 10/100/1000BASE-T RJ45 SFP transceiver and it will work just like a UE300.

Well, yes. That Realtek RTL8153 chip is basically a USB to Ethernet MAC-PHY so in a regular such adapter it is just that, an Ethernet to USB adapter. In the SFP case, the PHY is probably connected to another transceiver converting the Ethernet MDI back to SGMII resp. 1000-BASE-X which is what the SFP MSA mandates.

Not necessarily. I am currently experimenting with such a SFP modem giving me in excess of 500'000 kbits/s down and 100'000 kbits/s up.

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Well, for VDSL2, my current modem offers quite nice diagnostics all of which I would loose if I switched to one of the SFP form factor VDSL modules. Plus, I moved the bridged modem as close to where the telephone line enters my apartment, a position I would not like for my WiFi router, but moving the modem there eliminated quite a lot of interferences I picked up inside my apartment, so I do not want to move either modem nor router from where they are right now, one more reason not to play with SFP modules :wink:

That is a different beast altogether, but even there, I would happily live wit a bridged modem, if that allowed better diagnostics...