DIAGNOSIS Likely a transmit power setting or antenna orientation. The default transmit power is not always max.
FIX First, orient the antennas correctly. Do not point them all straight up. Position two vertically and two at a 45-degree angle. Log into the router. Go to Advanced > Wireless > Wireless Settings. Set Transmit Power for both bands to "High". Apply.
IF THAT DIDN'T WORK You might be on a crowded channel. Use the built-in scanner. Go to Advanced > Wireless > Wireless Statistics > Channel Utilization. See which channels are clearest. Manually set your bands to those channels.
HEADS UP WiFi 6 (AX) signal strength bars on older devices can appear weaker because the measurement is different. Test actual speed, not just bars.