T-Mobile Roaming?

I keep hearing that people with Sprint SIMs, who have compatible phones, and who haven't changed over to T-Mobile SIMs, yet, can supposedly use both the Sprint and T-Mobile networks.

I'm not finding that to be the case, though.

I've got an Unlocked LG X Charge phone that I know works on T-Mobile that's been on Boost for several years.

When I I use MTK Engineering Mode to disable bands 25, 26, and 41-- and leave the T-Mobile bands enabled, I only get 3g.

I'm not able to make it connect to the T-Mobile LTE bands.

Is anyone else experiencing anything different?

I'm not talented enough to disable bands (nor do have the courage to mess around with things like that!) but I had noticed that my Venn Mobile speeds improved substantially after the Sprint-TMo merger. In fact, back in April, the phone showed "Roaming" notifications. I assumed it was roaming on TMo.

I could never (edit *) get tmo when I was on sprint postpaid recently. Could never force it either. Apparently they will be switching people over to tmo primary in the coming months. Doesn't seem to be any particular order who gets to switch first.

*I did roam on tmobile for 30 seconds in the middle of the night once actually. Shame really as tmo is 100x faster where I am.