The transmission being stuck in 1 gear is an indication that it has gone into limp mode. If it resets after switching the ignition off & on again then this is because it is an intermittent fault & will drive ok until the car detects a fault once more.
If the transmission is going into limp mode then it should have stored a fault code - the first thing to do is get the codes read on a diagnostic tool.
Also the oil level should be checked - there is no dipstick installed from the factory so a special tool is required (a dipstick! These are available from tool suppliers or also on ebay).
Check the transmission for oil leaks - this transmission is prone to leaking from the electrical connector on the front of the transmission . If leaking from here then the electrical spacer & seals in the transmission will need to be replaced & the electrical connector cleaned with electrical cleaner.
A common fault inside the transmission is with the electrical plate - this usually shows up as a code for the transmission speed sensors. The valve chest needs to be removed for this.
Another possibility can be undervoltage fault codes stored – this is often down to a faulty battery. The battery should be checked with an electronic battery tester if undervoltage fault codes are present in the transmission ECU.
I would also recommend a transmission oil & filter change & inspect inside the sump pan for any signs of swarf indicating internal wear or damage to the transmission .
Best regards, Paul