I am unable to get landline internet here where my home is and I work from home full-time so I basically have 3 mobil broadband connections currently. In my current setup, I have a Cradlepoint IBR600-LPE broadband router with a Verizon SIM (my primary work connection) and I have a Netgear MR1100 (AT&T) in passthrough mode attached as the wired LAN connection. Currently I have both connections in a load balanced setup on the Cradlepoint using WAN affinity for some of the local PC clients or devices. I recently got a T-Mobile Home Internet router (the trash can) and the speeds and overall performance on it are way better than either my AT&T connection or my Verizon connection but, when I attach the TMHI router to the Cradlepoint, the double NAT is killing the speed to around 20 Mbit or less because the TMHI router does not have a passthrough option. What I've thought about doing was taking the Cradlepoint out of the loop and just load balancing between the MR1100 and the TMHI router (using something like my older ASUS RT-AC68U or even if I had to buy a device) but, I assume, if the packets were headed out the TMHI connection they'd need to be forwarded to it and if they were headed out of the MR1100 they'd need NAT added. Does OpenWRT (or some other router OS like Merlin) offer something like this ability?
Basically I just need a pointer in the right direction. I have managed switches and WiFi 6 Ubiquti access points, Raspberry Pi devices, etc., familiarity with OpenWRT and Merlin (although it's been a while) and I work in servers for a large enterprise so I have no problems reading and following directions for technical things but, I'm just not sure what I'd need to do this or if it's even feasible so I was hoping someone could tell me if it's possible and give me a brief overview of what it would take hardware/software wise and just point me in a direction where I could learn what I needed to know to set it up and maintain it.
Thank you to anyone that can spare the time to provide any insight or assistance.