So my primary router is a Linksys WRT1900ACS running OpenWRT kernel 4.19.76. Today, I used an old WZR-600 running LEDE 4.4.71 to successfully extend my 2.4 Ghz wifi using the steps in the user guide
I'm wanting to extend the 5Ghz more than anything, however the 1900ACS uses AC mode and the WZR-600 only offers N.
Is it possible to extend the signal on the same SSID and just have one operate AC and the other N? The steps say the mode must match, but didn't know if there was a workaround.
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.
Basically:
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.
Also the WZR-600 apparently supports OpenWRT19.07 (https://openwrt.org/toh/buffalo/wzr-600dhp), if you're really running LEDE still on it, you should upgrade. Plenty of bug and vulnerability fixes.
