I have the same modem/router and mine has been stuck on interleaved for what seems like ever.
The annoying thing is, the Asus syncs considerably quicker than either a HH5 or the OpenReach modem I have, but it stays stuck on interleaved.
I get a few CRC errors over a period of a couple of weeks (maybe 200-300), but BT refuse to acknowledge that there may be a line problem, despite the fact that the last time I had an engineer visit, they reset DLM and my speed jumped back to 74Mbps and a ping of 14-15ms. My landline usually displays a "line cord error" message a couple of times a week - and yet they say there's no line faults?
Roughly a day after the last Engineer visit, the HH5 that I was using at the time rebooted and I was back to what I have now ping 25ms and 67-68Mbps download.
My upload is fastpath and I ALWAYS get circa 19Mbps
To be honest, I've given up - I think that's what they want; to simply beat you into submission - lol