Wanted to follow up. C2100T modem is working perfectly well for days now. Issue was not dd-wrt or router, but CenturyLink's rental modem (specifically model C3000A).
Solution: Bought C2100T off of OfferUp.
(Had a tricky set up for transparent bridge, had to set up broadband page first with pppoe on, then turn on transparent, go back to broadband and manually set to tagged, then give quite a few mins and works.)
I'm literally in the exact same situation as you...I just upgrade to bonded a couple months ago (from 7 Mbps to 100 Mbps). I have the exact same router, the TM-AC1900. And I use transparent bridging. I however, have the Zyxel C3000z
Around the same time, I upgrade my dated firmware for DD-WRT...and immediately had problems...I failed forward and am now on v3.0-r43711 std (07/08/20)...when I realized it was not just DD-WRT, but the modem. I hadn't been sure, because the whole time, the modem always showed a good connection.
I finally found that the "randomness" was very infrequent, but to guarantee it...all I had to do was Apply Config to DD-WRT. At which point it became a game of rebooting the modem and re-connecting PPPoE about 3-5 times before all was good.
Dealing with CenturyLink to get a clear answer as far as alternative modems I can get has been a nightmare and I've been informed that the compatibility page isn't always accurate for the service...
Everything I was reading acclaims the Actiontec C3000a...but that's the model you had issues with?
How has it been with the C2100T? Does anyone have any alternative recommendations before I drop $$$ on a replacement modem?