I have had O2 at my current address off and on since 1987 (when it was Cellnet, with a huge Nokia transportable). I have always, even in 1987, had a reasonable O2 signal at my house. I had 12 months away from O2 until earlier this month, with BT SIM only, tempted by a very cheap deal but after a few months I came to remember the horrible service they provided when they used to be my phone and ISP supplier. The EE signal they used was definitely degraded, as I also have a French pay monthly Orange mobile on an identical iPhone SE. It gets a better EE signal at my house than the BT one does. I swapped the SIMs over, as both phones are unlocked, to check it was not the phone and it was not. I therefore decided to change to Tesco using the O2 network. The signal I am getting now at my house is disappointing, rarely more than one bar and often no service at all. Again I changed the phones over to check it was not a phone issue and it wasn't. I do therefore, wonder if there is some truth in the rumour that these secondary network sellers get a degraded signal or do not have access to all the masts. I rather wish I had bought direct from O2 but Tesco offered a SIM only bundle which suited better.
MVNO’s buy bandwidth from the main networks so will always be limited to what they have paid for.
Tesco, however, don’t usually suffer with this as they a a joint venture with O2.
I can confirm this. My other half is with Tesco and she generally gets as-good or better signal than mine (sadly have to admit it - Apple leads the way in mobile antenna technology - she always manages to get a slightly better signal than any of the Android devices I've rocked over the past few years :-))