@borgph
I have the same AX50 router as yours and you'll have to disable/turn off the malicious content filter of the AX50 for you to access that site you wanted. And just setup your own blocklist thru the Parental Control feature of the AX50.
You may have to use a PC/Mac to access the UI of the AX50 to make configuring the AX50 a bit easier. The Tether APP might have limited options.
The AX50 relies on the HomeCare or TrendMicro for the default block list of sites, and there's no way of adding a whitelist.
Cheers.