You will need to check to see the DD-WRT supported web page list. Even then, it does not always work and some times there is no path back. Follow the instructions if your router is listed or you can...

Attach your PC direct to the modem and see if you have any issues. Seems weird that both routers have the issue. I suspect it is your modem or ISP.

I do not know of any specific lists but the disk drive must be formatted either FAT, FAT32, or NTFS. The apple formats do not work. Also drives that require a lot of power may not work. You may need...

Does the guest networks have scheduling?

Reset the router to factory specs and reenter your data as it sounds like your router was hacked. Be sure to use different userid/passwords.

You may be able to if your router supports OpenDNS. See https://www.pcworld.com/article/3143456/networking/how-to-monitor-websites-visited-on-a-home-network.html