I am using a Linksys 1900 ACSv2 with OpenWRT (latest build from Divested-WRT: No-nonsense hardened builds for Linksys WRT series) as my router and have an AVM 1750e still with the original AVM firmware as wifi repeater. Unfortunately the mesh of the AVM repeater is only compatible among other AVM devices . In case I flash the 1750e with OpenWRT and then activate mesh, will it be compatible with the mesh setup from the router? Or is there a hardware limitation that also needs to be considered?
As a baseline -- the compatibility of mesh between brands (or firmware) depends entirely on what method the devices use for performing the mesh functionality. Specifically, there are some industry standards such as 802.11s, but some vendors will use their own proprietary protocols or they may implement the standard with certain tweaks that cause it to be pseudo-standard but not truly standards compliant -- this can be done to lock people into a specific brand and/or for technical reasons.
If both of your devices are running OpenWrt, there is a high probability that mesh can work because of the fact that you can run 802.11s on both devices, but there can be some issues from time to time that can apply when you're dealing with different wifi chipsets (which you would be here). IMO, it is worth trying, though.
. In case I flash the 1750e with OpenWRT and then activate mesh, will it be compatible with the mesh setup from the router? Or is there a hardware limitation that also needs to be considered?