UniFi AP AC Lite range/signal strength seems to be decreasing. How to fix/extend?

Hi. About a year ago I bought a UniFi AP AC Lite to provide WI-FI to the "granny flat" at the rear of my property. It fixed the problem, providing acceptable signal there.

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But now, the signal seems weak, offering 1-2 bars, and I'm told by my tennants it keeps disconnecting.

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It's well within the stated range of the product.

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1. Is there any troubleshooting I can do to extend the range? (Can range decrease with time?

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1. What are my options for extending the range with supplementary products if required?