There is a workout in the Xert library for getting a breakthrough and from that an accurate signature, go to workouts and use the search bar, a few will come up, there is a specific one for doing on a trainer. I wouldn’t use ERG mode for it as you want to be able to put out maximal power.
If you can ride outdoors a good test to generate a good signature is to warm up properly, then do 5 or 6 30 full seconds gas with 15 second recoveries, after the last one ride above threshold for as long as you are able, just as you start to struggle to hold the power do an out of the saddle all out sprint, this should result in a breakthrough and reset your signature. I’ve been using Xert for a while now and find it very good, takes a bit of getting used to, it’s a different approach, but it has paid off for me. They are very helpful and if you are struggling they will respond very quickly if you contact them.