That is not possible and there is no "workaround". WLAN is based on a very complex chain of techniques (frequency, modulation, bandwidth, antennas and a LAN itself). The WLAN part is defined in IEEE 802.11 and different from a cable based LAN. There you can find e. g. n, ac, g specifications. This specifications showing you how a WLAN has to be setup in a specific wireless environment (frequency, modulation, bandwidth, etc.).
Should be possible to setup WDS bridging mode. Because AC and AN are compatible, a 802.11an (5GHz) AP can connect to a 802.11ac AP as a client. Signal isn't repeated, more like forwarded between one device and the other via the WDS connection. You'll be limited to 802.11an performance however when moving traffic over the WDS link.
WRT1900ACS -> SSID: Main (WDS)
WZR-600 connects to SSID Main as a Client (WDS)
WZR-600 can then extend your LAN via Ethernet or via another WiFi SSID.
I had a TP-Link C2600 and a TP-Link WDR4300 connected like this via 5GHz as a test. It's a step up from simple repeaters in terms of speed.