It sounds like you're talking about DAC8 Stereo with a built in Raspberry Pi streamer add-on? I believe that's the only variant of Okto that has an RJ45 network connection.
How you/what app you connect to it with is down to what software's loaded on the microSD card of the Pi. I think it's a bit hands-on compared to something like the Avantree receiver you're using. Did the previous owner give any detail on what's loaded on there? Since it comes blank from Okto, you might need more info from the seller to use whatever image they setup.
It may well be the case there is an operating system installed on the RPi right now and you just need to put the Pi on a network and connect to it from a web browser on a device on the same network.
If the SD card is blank / it doesn't have a working OS here's some info:
RPi operating system info
Amirm's review instance came with
Volumio, however the manual states it comes from Okto blank.
In the same ASR review, it's suggested to load it with the Roon endpoint for full 24/96 support, that info may be outdated now however. From some brief searches it looks like it could be loaded with
RoPieee a "ready to go" Roon endpoint image. Although the
FAQ on the Okto website says there are some caveats to using with Roon. Other users on other
forums talk about RoPiee XL working well inside their Okto 8 DAC stereo.
I can't speak to how you configure the Wi-Fi credentials on all the various RPi images available out there, I assume you probably embed it in the SD card image when preparing it. It can vary. For Volumio for example, it creates a temporary wireless hotspot that you can connect to, then in the web browser of the connected device, configure it to connect to your proper home Wi-FI. This is explained in the
Volumio quick start guide, page 5 onwards.
Wireless antenna
If you're without the antenna, it does appear to be a generic ones available widely. The
DAC8 manual suggests not removing this antenna even if the users going purely wired, because it's removal could lead to audible RFI issues, might be sensible to replace it if it's missing.
See 5th image in gallery of this amazon item, it's identical to that in the DAC8 Stereo product page.
https://www.amazon.co.uk/Waveshare-Compatible-Raspberry-Supports-Frequency/dp/B08RRX9H2Q
USB Jumper
From amirm's original review, it looks like the jumper (or "USB interconnect" as the manual calls it) is just connecting the DAC to the Pi. Similar to if a PC was plugged into USB 1 just next to it. It looks like any cable could suffice for this if that's missing. I'm not sure how the previous owner was using the RPi element without that jumper installed, it's how the RPi sends audio to the DAC.
I would definitely reach out to the seller on what SD card image they setup previously.
Cheers.
