Get a second access point installed.
However, if you try using homeplugs they will be on a different ring (downstairs and up) and that can cause them not to communicate or run at low speeds. I personally do not like them and will almost always recommend and alternative.
Is there any way you can run a long Ethernet cable between the two locations? If you have carpets it is normally possible to squeeze the cable into the gap at the edge of the carpet and under the skirting boards. so will be hidden for most of the run. Or even get a couloured one to blend in with your paint work and clip that to the staircase string.
M H C
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