[While a good suggestion in general, mixed-mode on the WAP has nothing to do with this problem.]
Even though I have upgraded my wireless routers many times over the years, and now have a super-fast, long-range, 802.11ac router, I also wired my two-story house with Ethernet cable between floors, (back in the 802.11g days). It's really the best solution, if you can somehow run a cable between your router and your phone. One way to accomplish this in an existing construction environment is to install (or hire an electrician to install) a single-gang wiring junction box in the outside-facing wall on each of the two floors, and then use PVC conduit along the outside wall of the house to go from the two boxes, though the walls. Painting it to match the house paint makes it practically disappear visually.
By the way, if you use Ethernet cable for the OBi phone, you can also get rid of the AC/DC power adapter, and use Power over Ethernet (PoE) instead, either via a new Ethernet switch with built-in PoE, or use a PoE injector for that cable run.