What does it mean when you say "唔夾"? mean while working with other devices in wifi? or problem in lan connection?
AC68U is using Broadcom chipset. If it is the problem of compatibility of your device with Broadcom chipset, you should look for other router which is using other CPU/Chipset. (R7000 is also using Broadcom chipset which potentially would just repeat the same issue that you have)