DIAGNOSIS This is usually the "Smart Connect" feature doing its (sometimes overly aggressive) thing. It tries to steer devices to the "best" band, but some older devices don't handle the kicks well and just latch onto 2.4GHz.
FIX Log into the Linksys app, go to Wi-Fi Settings, and turn OFF "Smart Connect." This will split your Wi-Fi into two separate networks: YourNetworkName_5GHz and YourNetworkName_2.4GHz. Manually connect your faster devices to the 5GHz SSID. Problem solved.
IF THAT DIDN'T WORK There's a known firmware bug in some older Velop AX versions that can cause 5GHz instability. Go to the app's Firmware Update section under Router Settings and check for an update. If one's available, install it. The updates often include stability patches.
HEADS UP Splitting the bands means your devices won't roam automatically between them as you move around. For most people, that's a fine trade-off for stable speed.