New local wifi network (T-mobile) - connection to volumio not working on pc, android, or iphone

Running Volumio 2 on Raspberry Pi 3 for years using ATT internet. Recently switched to T-mobile.
Tried to get Volumio working again.
At first it looked like it would work. It started the hot spot and I was able to connect to it.
Volumio would play music using hot spot access.
I set the network up to connect to T-mobile wifi.
I switched my Win7 pc to connect to T-mobile wifi and couldn’t connect to volumio (volumio.local)
I switched back to hotspot to verify that volumio was actually connected to T-mobile.
I re-entered the password (even though it was done via copy and paste and was correct.)
This time it “appeared” to work.
I switched my pc back to the T-mobile wifi and tried connecting to volumio. No connection.
I tried several times using http://volumio.local and after about 6 tries it actually connected.
Happiness for once.
Then I tried to use an android phone to connect to volumio. Installed the volumio app.
Connected to the T-mobile wifi. Volumio app couldn’t find volumio. Tried using browser
and volumio.local. No connection.
Tried on an iphone using T-mobile wifi and browser using volumio.local.
No connection.
Went back to my pc and the browser was no longer connected to volumio.local
and it wouldn’t connect.
Tried to switch to hotspot and no hotspot wifi was available.
So I switched off the RPi and restarted it.
It appeared to work and the volumio RPi system booted up.
It gave the volumio ready tone on my speakers and it appears ready on the
RPi display monitor.
But no connection using T-mobile wifi and no hotspot is seen from pc or phone.
I have no access to volumio using wifi and don’t know what to try using the command line.
I have no idea what to do now to get this working and stable.
I’d appreciate some help or links to instructions that I can use to debug this issue.
Thanks in advance.
David

Volumio 2 has been unsupported for a number of years and you will have difficulties finding someone to help.
None of the regular community supporters deal with Volumio 2, so I suggest to upgrade to volumio 3.

Thanks for your reply @gkkpch. I have been thinking of doing that.
Should I expect any issues using ver 3 with RPi 3?

no it runs fine

I have installed the latest ver 3 and my android can see the RPi volumio.
I set it to the correct wifi network.
Both my pc (volumio.local) and the android volumio app do connect to it…
I selected the same I2S playback option (HifiBerry DAC Plus) as I used briefly with volumio2 2 days ago (and that has worked for 5 years.)
It worked great with volumio2. It doesn’t work at all with volumio3.
I get no output via I2S. I do get output via HDMI.
I am using an inexpensive DAC bought on eBay 6 years ago:

I tried another dozen playback options but no output.
I tried DSD Direct and DSD over PCM with the HiFiBerry DAC Plus. No output.

Can someone help me get this to work with volumio3 as it did so well with volumio2 for years?
Should I repost this issue under a different subject now?

Can you select your dac in settings reboot and send log please?

Thanks for your reply @balbuze.
I started to look into getting the log for you and when I booted up volumio and set the I2S to HiFiBerry DAC Plus, volumio3 now works correctly and I am getting output from I2S DAC to my preamp.
So, problem is apparently solved, fingers crossed. I waited to respond til after running it
a second day with success.
Thank you for your help.