@rwaldron and @HipsterBrown Thanks both for your replies. I had to retrace my footsteps, as I made no notes before, and have made some progress.
All I have really done with the Tessel 2 so far is connect it to my home network, and run some basic code examples using t2. Then I tried to use it as an access point, following the instructions at https://tessel.github.io/t2-start/ap.html. The problem was that I could create an access point and connect to it with no problems, but as soon as I was away from my home network, or switched it off, the access point became invisible. Turning the access point off and on again with t2, or even creating a new one, did not change this.
After reading the information at https://github.com/tessel/t2-cli/issues/755 I got the feeling that my issue may also be with the file
etc/config/wireless, and when I replaced my file with the version from GitHub, I found my access point was now working.
To confirm the source of the problem, I connected the Tessel to my home network again, and the access point again became invisible. As long as all I have to do is switch between two versions of the
etc/config/wireless file to either connect to a network or use the Tessel as an access point, I'm not too bothered, but should this be necessary?
By the way, when I tried to run
t2 wifi --off, to see if that would be a quicker method of getting my access point back, I get the error:
Error: Missing Wifi State: (empty). Could this be a connected issue, or am I making a mistake in the command?