I donāt think Monzo have gone complicated by doing O2 for domestic and 1global for roaming, itās most likely 1Globalās white label telco
Theyāve done N26 recently, and N26ās only 3 plans 10GB/30GB/100GB are suspiciously close to Monzo.
According to 1Globals docs (which do look a bit old) they have 9 core markets but only 6 point of presence, luckily one is the UK so we should be getting UK IP addresses.
Most of these notes are just nerd stuff and 95% of users will be happy but using a full stack MVNO means you just donāt get the full feature set of an MNO with the phone OEM support but millions of people are happy on Lebara/lyca/sky etc so it should work well for most people.
Looking at N26, they use the 1Globalās APN, personally I donāt like this as it introduces more points of failure over using O2 directly.
N26 has various data add ons available from 1 to 20GB which is nice to know the option is there.
I just suspect teething issues like Spusu with texting shortcodes and premium rate and geo-ip being wrong, some things are off by default and N26 docs say call forwarding itās billable, it could be different in the UK market but I think itās safe to say itās not a raw O2 service and there will be gotchas. New technologies like RCS and HD calling will be slower to arrive.
Also N26 international rates are not great, could be something with German number interconnects so weāll see.
There is missed calls texts.
Surely this answers Carlo if SA is included 
Radio Networks in 1GLOBAL Countries: In each 1GLOBAL country, 1GLOBAL partners with a large incumbent Mobile Network Operator (MNO) in order to ensure the best possible radio coverage. Coverage provided to 1GLOBAL customers is no different to that provided by the operator directly to their end customers and the availability of radio network coverage will also be the same.
To be clear itās unlikely SA is included imo.