The result is as expected. The ALLNET DSL modem runs stably without disconnections and the connection quality has improved markedly (internal DSL modem of the Fritz! Box is disabled, the DSL connection is established by FRITZ! Box made via external ALLNET modem). The configuration of the ALLNET is perhaps not quite as intuitive as with the Fritz! Box, but after a short training certainly doable.
I can ALLNET DSL modem only recommend.