The issue may be your devices rather than the router. They are not the best clients for range or throughput because they are usually one antenna and very low power optimized.
Which firmware are you using?
The RT-AC68U is a step up from your router, especially in the 5GHz band. Recommend the RMerlin firmware, latest is 376.48.1.
Have you tried creating a new ssid on the router or forgetting and / or resetting the network settings and then re-associating all your clients?
Have you tried different channels? A better location for the router? Higher and more central is always preferred.
Have you ever reset the router to factory defaults and then manually enter the minimum needed to get connected to your ISP and to your clients?