Does it mean that even if they fix it for [A], it may become slow for [B] since it's hit-or-miss? Unless they can assign a static IP address for the modem?
Yes, that is correct assuming that the static IP address is associated with the faster segment. BUT a static IP address does not guaranteed that you will be on the fastest segment because their networks are constantly changing [adding customers etc.] But with the right infrastructure they can control how speeds [bandwidth] is managed for each of their segments --- so it all depends on how they [Rogers] are modernizing their equipment for your specific area.
So yes, you just have to be patient and hope that the Rogers infrastructure that manages your area is being upgraded to deal with the kind of issue that you are experiencing --- at some point it will balance itself out.
The technician that will be coming to your home [to analyse] will report his findings to Rogers and Rogers will try to make adjustments to fix your issue assuming Rogers do have the upgraded systems in place. -- David Mozer IT-Expert on Call Information Technology for Home and Business