I have been using the TL-ER5120v4 for a number of years with my Gigabit Internet and with an LTE/4G router as failover. It worked very well but I decided I didn't need the LTE/4G connection any longer so disconnected that router from the TL-ER5120v4 and removed the config.  It all worked well with just the one WAN connection but then I started getting what I thought was "internet outages" to start with. On further analysis I found that I wasn't losing Internet Access but DNS for periods of about 20 minutes at a time. If I was using a PC that was already using a web browser then the issue never happened on that PC, however if a different PC opened up a web browser then everything failed with a 404 error.

When that fault was occurring I started doing some further analysis. I couldn't ping any public domain name from the LAN devices however I could ping a public IP address. If I used the tools section on the TL-ER5120v4  web gui then I could ping any public domain whilst any LAN device couldn't. The fault would eventually resolve itself after 15 to 20 minutes or so and I would then forget about until it would happened again a few days later. If any device used 8.8.8.8 or other DNS server as it's default, then the issue never happened to that device. If I added 8.8.8.8 as a DNS server to my WAN config then the issue did still happen.

I eventually removed the TL-ER5120v4 from my network and the issue didn't occur again with the replacement router.

I have started live streaming in the filed and use two routers with 2 different mobile networks and decided to try the TL-ER5120v4  to load balance them. A bit over kill maybe but when I tried this yesterday the DNS issue occurred when the laptop streaming didn't have an issue but another PC couldn't connect to any public domain for a while.

I had perviously read that the TL-ER5120v4  can detect DDOS and large pings/ping of deaths incorrectly, so I did previously remove all that firewall protection for a bit a few months ago when I was still using the router at home, but it made no difference. My summary is that for some reason the TL-ER5120v4 stops resolving any DNS address for any LAN device for a period every few days whilst the tools section on the GUI still resolves them.

Any thoughts or do you think it's time to replace it with one of the other gigabit load balanced routers such as a TL-R480T+ TP-Link TL-R605?