T-Mobile Home Internet Slow IPv4

You mentioned you switched to using external DNS, and that it may be related as it coincided with when you started noticing things were getting more laggy.

TMO HI is using only a v6 layer between their modem and the edge networks, which is basically like a CGNAT scheme on the forward facing network segments.  Their is no 1:1 v4 path in or out--so there can be some wonkiness with forced v4 only traffic...  especially for things like DNS queries and ping/trace tests.

The whole system works kind of like a VPN tunnel... more specifically, a sort of v4/v6/v4 tunnel.  One of the more egregious things with this setup is how it monkeys with inbound v4 traffic.  Any unsolicited inbound traffic gets killed at the edge network because the weird sort of VPN scenario going on... without the proper tracking that happens with an outbound request, the system has no way to forward unsolicited traffic to your modem (basically, this is why port forwarding and such is hosed so badly atm).

Our traffic in general is also set at a lower priority... they do this to preserve more time slices for the phones.  And that gets doubled down on when you are running ICMP ECHO requests for testing, as the networks generally place such requests to lower priority as well (if not set to outright ignore them, especially once utilization crosses certain  thresholds).  Couple this with trying to force v4 pathing... and things can get weird.

In short... by trying to force v4 communication, you are opening yourself up to a lot of potential issues.

Take a close look at the Network/Status page of their Askey modem... particularly the v4 and v6 sections.  Notice how we are forced to v6 addresses for everything upstream?

The modem is set to push all data through a v6 gateway.  It's DNS queries are run against V6 servers.  Trying to force v4 only and such is kinda asking for weird things to happen.

Your local network and devices need to be set up in a way so the routers/modems can try to forward it all as they are set to do so natively.  If a v6 path is viable, it will use that, otherwise reverting to v4.  Note that some browsers may misbehave though and may need tweaking for v6 to work more reliably. 

The Askey is mostly configurable and will TRY to function just like any other router (minus an actual bridge mode selection).  You can set your own LAN subnet up, so you are not stuck with the 192.168.x.x address it came with out the box.  Don't even remember their default to be honest.  I changed it to match the subnet my Netgear modem used when I was on Spectrum.  This way everything on my side stayed the same way I had it before, even my shortcuts to my router and the modem. Literally plugged it all up, rebooted my Asus to make sure it synched to the modem properly, and was off to the races. I tinkered with other confiigs, but reverted to my Spectrum model.

Basically, either run your router as an access point or normal mode and double NAT it... but let things configure DNS automatically or otherwise manually forward the first entries to the appropriate gateways so it ultimately gets directed to the TMO v6 DNS servers FIRST.  THEN the external v4 lookups can kick in per client if the initial query fails.  Things just  tended to work more reliably when my Asus configures/forwards DNS automatically versus manually pointing it to the TMO gateway first and then Google secondary.  I have some clients manually configured to look to the Asus as primary DNS, the TMO modem second, then Google and such as additional entries after those two.

An example run down with a router in normal mode (will Double-NAT locally):

Router automatically sets up the WAN uplink against the Askey LAN1 or LAN2 port via DHCP on the Askey.  You can leave the Askey with it's default LAN subnet, or configure your own like I did (192.168.100.1 root, just like it was when I was on Spectrum).  Run DHCP on your router for your LAN just like normal, be that it's default or again a custom one like I already had set (I used the 10.10.x.x space).  This will allow you to keep your static bindings and such as you had before.  Your devices should be looking to your router for gateway, DHCP, and primary DNS, with your router forwarding DNS to the TMO modem/router just like it would by default with your prior ISP.  Then the TMO device runs as your router's gateway, and forwards all the DNS queries to it's v6 DNS on the other side.  Only the necessary v6/v4 translations will happen by design without any of the extra wonkiness that can creep in when you try to force things to run differently.   

Should note that you can set up v6 passthrough on your router if you want.  Trying to setup a v6 delegation may appear to work, but tends to break shortly after.  Setting v6 passthrough on your router and enabling v6 on your devices will kinda sorta work.  It wasn't the greatest, but at least the phones seemed to like it... somewhat.  V6 is always a bit screwy when it comes to Windoze.  The PS4 tries v6 if it is active on the LAN, but Sony never did implement v6 right, so it misbehaves and falls back to v4.