And I have already checked all the simple gotchas such as https://www.imobie.com/support/how-to-fix-cellular-data-issue-on-iphone.htm
After getting some logs from the device, the following seems very suspicious:
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: NSURLSessionTask finished with error - code: -1009
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I New CTServerConnection from pid 4515 (conn=0x1076efb80)
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I CTServerConnection from pid 4515[<private>] is named '<private>'.
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: HTTP load failed (error code: -1009 [1:50])
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: NSURLSessionTask finished with error - code: -1009
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I CTServerConnection from pid 4515 has closed (conn=0x1076efb80)
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Notice>: TIC TCP Conn Start [42:0x17019ce50]
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I Got denied callback for cellular when cellular is allowed
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I Ignoring access denied callback for bundle id: <private>
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Notice>: TIC TCP Conn Event [42:0x17019ce50]: 3 Err(50)
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: TIC TCP Conn Failed [42:0x17019ce50]: 1:50 Err(50)
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Notice>: TIC TCP Conn Cancel [42:0x17019ce50]
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Notice>: TIC TCP Conn Destroyed [42:0x17019ce50]
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Notice>: _CFNetworkIsConnectedToInternet returning 0, flagsValid: 1, flags: 0x40000
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I New CTServerConnection from pid 4515 (conn=0x1070789c0)
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I CTServerConnection from pid 4515[<private>] is named '<private>'.
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I CTServerConnection from pid 4515 has closed (conn=0x1070789c0)
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: HTTP load failed (error code: -1009 [1:50])
Jun 7 11:08:23 iPhone advomobo(CFNetwork)[4515] <Error>: NSURLSessionTask finished with error - code: -1009
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I Got denied callback for cellular when cellular is allowed
Jun 7 11:08:23 iPhone CommCenter[81] <Notice>: #I Ignoring access denied callback for bundle id: <private>
what could be the cause of this issue?