Personally, I do not like Extender mode in OpenWRT because it uses relayd, which kind of interconnects 2 subnets. In this way, there may be firewall acting between them. If the router will be dedicated to the 2 astronomical devices, then I would try doing a factory reset the router and set up the Extender mode again in case there is interference from a previous setting. It should be okay to have a different wifi SSID from the main router, which many extenders allow, or you can turn off the extended wifi via LuCI → Network → Wireless.
It is worthwhile to try Router mode and just add Port Forwarding from WAN to the devices on the required ports. I prefer port forwarding over DMZ, which is similar for ALL ports and is only for 1 device/IP address.