Glad you managed to identify the issue.
Also, for information, there seems to be misinformation about Fritz's Annex A and B, some people wrongly believing that Annex B should be chosen on VDSL lines. So, for all exchanges (Broadcom and Infineon/ECI), it should be Annex A on the Fritzbox. The only time when you should choose Annex B is when your cabinet has the Ikanos chipset.
However, if you choose Annex B for Broadcom/Infineon, Fritzbox will ignore this setting and treat it as if it was Annex A, which is why probably some have been successful with running their connection with Annex B setting - still unaware of the difference - thinking/being convinced that this is the correct setting.
Such Fritz boxes are still aimed for the international market (POTS).
Confusingly, Annex B also refers to ISDN, which is what is used in Germany and Poland, but not by all providers, which adds to the confusion. In such cases, modems are identified as working with specific providers, for example modem working with Orange or 1&1 or Deutche Telekom in Germany, or TPSA or Dialog in Poland, rather than modem for Annex A or Annex B.
Therefore, if you order telecoms equipment from Germany especially (in relation to Fritzbox), you have to pay attention what market it is for. If it's for the German market, it won't work in the UK. If it's for the international market, it'll work in the UK. Then, you can choose 'Annex A' or 'Annex B' in Fritz's settings, depending on the chipset installed in your cabinet (Annex A for most).