Not enough info from you - are you using an OpenVPN client in your router? - so I'm just going to toss this out there in case.
When I first set up AirVPN via UDP on my dd-wrt router, my connection tended to hang now and then and require a restart. It turned out to be that packets were occasionally too large for my ISP's network. This is actually a very common problem when you use a UDP vpn connection, and you fix it with an "mssfix" command in the OpenVPN config.
So first, see whether your config already has an "mssfix" command (with or without a numerical parameter), has a "fragment" command (which also sets the mssfix parameter), or has "Tunnel UDP MSS-Fix" checked in a dd-wrt config. Eliminate any of those to make the test clean, and try adding "mssfix 1300" to your config to see if that fixes things. This tells OpenVPN's clients in your system to please limit packet size to 1300 bytes.
The 1300 number is almost certainly lower than necessary and will slow things down a bit by forcing clients to split packets, so if it works for your system, you'll want to find one of the many online explanations of how to determine what the maximum packet size is that your ISP's network, can handle. I found https://www.sonassi.com/help/troubleshooting/setting-correct-mtu-for-openvpn useful. Do the test through the vpn tunnel, again with "Tunnel UDP MSS-Fix" unchecked and no fragment or mssfix command present in the config. Here's my own linux/bash version of a test:
for n in {1450..1300..-10} ; do
ping -q -M do -s $n -c 1 mtu.edu >/dev/null 2>&1 && { echo mssfix $n; break; }
done
For me the "mssfix 1410" config command that this scriptlet suggested solved my problem most of the time, but I eventually had to drop it to "mssfix 1400" to completely fix things.
Remember there is no one answer. It depends on your ISP and may vary some from one day to another. But 1400 seems to work for a lot of people. Again, this is not some obscure thing affecting nearly no one. In fact this is an extremely well-known, common issue and worth some experimenting.