Why is the Wi-Fi fine at my desk and terrible in the back?

Two different causes that look identical to the person experiencing them.

There is no coverage back there. Signal falls off faster than people expect, especially through a wall with plumbing or electrical in it. Your laptop shows one bar and technically connects, which is worse than not connecting at all because applications hang rather than fail cleanly.

Or your device will not let go. Laptops and phones are stubborn about roaming. They cling to the access point they first connected to, even after walking well past a closer one. This is why the Wi-Fi is fine if you start your day in the back, and awful if you walk there from your desk.

A quick test that tells us which: turn Wi-Fi off and on again on your device once you are in the bad spot. If it becomes fine immediately, it is a roaming problem and it is fixable in configuration. If it is still bad, there is a coverage gap and you need an access point back there.

Both are solvable. Neither is solved by moving the router.