Along the line of Tim's point, make sure your endpoints (the cameras) are connecting to the intended edge AP, and in turn, your edge AP is properly connecting back to the base AP. Assuming that is true, I would try re-working your radio layout to move your broadcast blanket closer to the place you're having trouble with. That will allow you to use lower-power signals and have higher throughput with less errant signal.
If relocation isn't possible, would it be possible to somehow add a wired connection for your remote AP through ethernet (first choice), MoCa (second choice) or powerline (typically last choice)? If so, you could hopefully move the edge AP closer to the concrete as well and even forgo wifi repeating altogether, which would give much better backhaul bandwidth, lower latency and a freedom from any of the myriad of LAN issues than can be created from generic repeating.
If you can't alter your wired infrastructure or relocate radios, then a swap for a better class of radio and more radios is your next and probably last option. The most simple first attempt is via a bigger/stronger all-in-one to serve as your base AP (R7800, AC-86U, etc.), seeing if its signal alone may be enough. If not, then an AP or all-in-one with dual 2.4Ghz and 5Ghz radios would be desirable as your extender. If AP link distance is long, you might want to use 2.4Ghz for that, then 5Ghz for your endpoint connections, or vice-versa, depending on what performs better.
