The Asus RT-AC56U is just a very bad choice for running OpenWrt, as it ships with two Broadcom softmac wireless cards, which are barely supported at all (b43 doesn't support HT/ VHT modes, which means it's limited to 54 MBit/s). This is not going to improve in the future, as Broadcom has always been rather hostile towards opensource driver development for their softmac designs.
Yes, you can connect external APs to your Asus RT-AC56U, however any decent AP (depending on your needs, ipq40xx, ipq806x, mvebu) worth its salt could take over the routing tasks of the Asus RT-AC56U as well, making it optional (replacing it alltogether).