If O2 have confirmed no restrictions you should maybe check if Boku have any limits set.
This is from a casino that uses Bokus platform,
Most casinos in the UK set their minimum deposit limits to a small sum such as £10.
So contact your online casino.
yes that "minimum" I've used boku with my account under ee and there was no issue. reply from boku:
Reviewing our records, I see that your recent purchase attempts were not successful. The transactions failed because they exceeded a limit set by your mobile carrier. Once a limit has been reached, no further purchases are permitted until it has been reset.
To address this issue, I suggest taking a break in between transaction attempts. You may also wish to contact your mobile carrier for further discussion of the limits on your account
I've changed my cap limit from no cap to £200 also and it's still not happening.