The MT50 will show connecting if it is getting power, but not successfully getting responses to its requests to the solar charge controller (SCC). That could happen for a number of reasons (in order of troubleshooting process);
Bad cable (Try a different Ethernet cable)
Bad connection at either SCC or MT50 (Visually inspect pins; if damaged - and skills allow - repair)
Something logically wrong with SCC or MT50 (Power off and fully disconnect both devices for a few minutes then reconnect and try again)
Something physically/electronically wrong with either SCC or MT50 (Probably requires replacement)
The MT50 is cheaper so that could be a first device to replace if optimizing for cost efficiency when troubleshooting
Otherwise, having a backup MT50/SCC can be handy so ordering both isn't necessarily a waste long term and will improve time-efficiency troubleshooting