The rx4-1500 had several vulnerability issues with firmware versions 1.0.2 thru 1.0.5, and one developer did a thorough report on it several years ago before Juplink dropped all support for the unit. I contacted him about the report, and he sent me a version 1.0.6 that he obtained directly from Juplink before they discontinued support, which eliminates the telnet option vulnerability.
I upgraded my rx4-1500 to that version (I previously had V1.0.2). I'm very happy with it, as the 1.0.2 lacked many features found in later firmware versions. Version 1.0.6 is available here:
https://drive.google.com/file/d/102sdwrrUGkJUGPzGAAcALAGSrYZ2HCqe.
The only issue I found with the new version in my use of it as a bridge wireless router (with DHCP disabled) was that the static IP address I preferred kept changing upon reboot (this was never an issue with 1.0.2). However, this problem could be through some faulty settings on my part. I solved the issue by assigning it a static address via my main home router.
Also, if you want greater control over the unit, there's a folder /bcm that contains far more extensive controls. You can access it on the router via http://ROUTER_IP_ADDRESS/bcm/main.html