Hello @Mazuak ,
I had similar issues you describe on a Fritzbox 7520. For me there was a patch needed to bring the DSL interface up correctly. A hint for this can be found in dmesg:
[ 11.432031] vrx518_tc:pcie_ep_probe: Total 1 VRX518 EP detected
[ 11.444606] vrx518 0000:01:00.0: dc_ep_clk_on failed
If that is found on your device there is also a patch needed. I tried a lot different configurations but all ended up in this initialization loop you described.
Finally @janh provided a patched version which did get a DSL link with the configurations as described for 1&1 DSL.
Here is the full forum post of my tests: