No blaming or shaming intended.
Openable ferrite clamps on the phone cable may help reduce the EMI coming from surroundings. That won’t surely transform the cable into a shielded one, but… if you can find some spare ones even from old VGA Cables… who knows?
If you can’t access to the xDSL page of your xDSL router you cannot find the attenuation value and the SNR value, which can help you understand better if the connection between your CPE/xDSL router to the DSLAM (the ISP counterpart) is wonderful, nice, not that bad or crappy.
Also… in 30 years sometimes ethernet devices are not that compatible as they should be. Beefy corporate-grade switches can more or less eat/connect everything as negotiation (sometimes with some settings on ports) and can manage quite well crappy 8pin Ethernet cables. But consumer grade devices and network cards sometimes cannot keep a stable relation between the two sides. So try with another network cable between IPFire and the CPE/xDSL router might be something cheap and easy to test for change something.
Also… If the connection is not that fast (less than 100Mbps down or up) you can think to downgrade the network negotiation between CPE and IPFire box, if changing cables don’t work.
I’m bored, so i’m willing to share these simple tests for trying to troubleshoot your issue… but it’s quite annoying to read for the… maybe 1Mth time than “culprit is the new toy” only because the previous one (software, device, consumer device, whatever) was not refined enough to show the problem. Few months ago i were quite pixxed off with my smartphone because at one customer site the WLAN was wobbling up and down as connection, while the crappy old laptop did not. Long story short, the WLAN adapter of the Access Point was really unstable. The old 802.11N wireless card of the laptop didn’t budge, the 802.11AC adapter of the smartphone showed the db level graph shaking like and earthquake chart.
Shame on me once to don’t investigate before cursing my device, shame on me twice for assuming that the problem was “the new toy”. It only showed better the truth (after the AP replacement, also all the WLAN connected AC units started to work flawlessly).
A little hint for the next IT/net shipwreck: when something’s not working as expected, don’t blame the new thing, do some homework and verifications before