Okay. On the basis of this additional information, your problem is now fully understood. I can replicate the issue with a BT Business Hub 5. I suspect you may be using a BT HomeHub 5?
In addition to the controls that you illustrate, this issue is evident in most (if not all) configuration pages that use this menu/selection feature - including the system logs.
The problem you describe is associated with the most recent iPadOS/iOS 13 update - during which there was a substantial update to WebKit. WebKit is a key component that influences the way that the browser(s) on your iPad/iPhone both render and process web pages and page elements.
Very often, it is possible to workaround WebKit issues by refining advanced settings of Safari - but for this issue, unusually, I’ve not been able to identify a fix for this explicit issue. Using an alternative browser with your iPad also can’t solve this particular problem; in most cases, third-party browsers on iPad/iPhone, there remains a dependency upon the WebKit component.
Browser problems can be also often be worked around by switching iPadOS Safari from Desktop to Mobile browsing mode; in this instance, this option is also ineffective.
For the problem that you describe, there is no currently no direct on-device workaround. For the time being, I can only recommend that you use another computer that is not affected by this issue (e.g., PC, Mac or iOS12 device).
I remain hopeful that a future update to WebKit might resolve this issue - but this is obviously not guaranteed. Whilst an inconvenience, the issue that you describe appears to be relatively isolated.
I’ve only identified this particular problem with devices, such as Routers, that incorporate their own web server. Similarly, a future firmware update of the Router might similarly resolve the problem. Such an update will likely be contingent upon a sufficiently large number of complaints being directed to the appropriate technical support channel - in your case, BT.
The only other advice that I can provide is that you report the issue - including technical details of the installed version of iPadOS - to both BT and Apple Support. Be mindful that the issue will not be seen by a large number of users, therefore, the issue may not be prioritised.
Whilst I can’t at this time offer anything more by way of practical assistance, I hope this information and insight is helpful in confirming that the problem exists - and that there is nothing at this time that can be done to immediately correct the issue.
Postscript: The Hub5, from a control perspective, is a far better router than the so-called “Smart Hubs” (a.k.a., “Hub6”) that replace it - particularly in context of its DNS implementation and controls.