I've run a Superhub 2 in modem mode for eons.
And just 3 weeks ago, because I was setting up monitoring on my mothers home network so that I can be alerted when there's a problem (instead of waiting until my next visit, at which point she casually drops into the conversation X isn't working), I wanted to put her network into an IP range I don't use anywhere else.
Steps:
1) Buy a TP-LINK Archer of some description, not dissimilar to yours
2) Plug the TP-LINK Archer into a power supply
3) Connect my laptop to the TP-LINK Archer via ethernet (wiFi disabled) and configure the SSID/password/IP range
4) Unplug my laptop from the TP-LINK Archer and plug it into the Superhub 3
5) Connect to the web interface on the Superhub 3 and put it into modem mode
6) Unplug the laptop from the Superhub 3
7) Plug the WAN port on the TP-LINK Archer into the right port (only one of them works) on the Superhub3
9) Enable WiFi on the laptop, choose the new SSID and type in the password - check connectivity
It just worksTM
I see that you have 24 Wi-Fi clients. This is at the top end of what I've seen consumer grade access points do. I suspect this might have been a factor in your Superhub WiFi being problematic - at least pop WiFi Analyser onto your phone and ensure you are using Wi-Fi channels with least contention with other Wi-Fi networks. If possible, evenly distribute your devices between 2.4 and 5Ghz.