Since RUTX50 only has a single modem, neither Failover nor Load Balancing can be used between both the SIMs.
In this case, you will need to use SIM Switch. As for the weak signal threshold, there are no "correct" values, and it's basically a guessing game. Since you are connected to LTE B28, this band is used for covering wide areas, so I'd say a weak signal for this band could be -80. But for bands B1 and B3, this value could be much lower.
I noticed on sim switch on "data connection failed" using ICMP constant switching between the two providers whilst one network was stable as opposed to the other
Could you try pinging the specified address on the carrier that keeps failing by navigating to System → Administration → Troubleshoot and scrolling down to the page. This will let us know where the issue could be. Also, make sure the timeout is set high enough.
Should I also use Network>Failover>with the default rule for 0/0 also!? Do the two conflict Sim Switch & Failover or I should use one only!?
SIM Switch should be used. Failover or Load Balancing can also be utilized if you also have wired WAN.
Also can the RUTX50 do some kind of latency/packet loss failover on data or is this feature to be introduced in the future?
On data connection fail parameter basically checks the same thing. If ping packets are lost, the SIM Switch is initiated.
Under Network>General>Low Signal Reconnect what does this do?
This option reboots the modem in case a very bad signal is detected, in hopes that the modem will connect to a different cell or mitigate other issues.
Let me know if you have any additional questions!