From another perspective:
As this is your secondary line, and the most important thing here is probably more data bundle, TPG 50GB $10 plan may not be a bad deal.
It is true that service coverage in tunnels is still work in progress, and inbuilding coverage in non-commercial buildings may still have issues.
However, as the bundle is 50GB/month, vs MyRepublic's 6GB (exc throttled), TPG may not be that bad. It is not unlimited, but with 50GB, as long as you don't use it to download large stuff on the go, it's practically eat-all-you-can buffet.
While inside tunnel, just switch back to your main line to normal stuff that you're used to now. Likelihood of exceeding the primary data bundle should be much lesser.
Sometimes inbuilding coverage woes do happen on non-TPG networks as well, it is not that the incumbents are immune to this problem.
Also, there are stretches in the train tunnel that M1's network do face network congestion during peak hours. Even if you have service inside, sometimes you just see spinning circle as well. So it's not all sunshine when using M1's network anyway. (geddit? Sunshine? M1?)
Btw, I'm using both M1 and TPG. So giving this perspective doesn't mean I'm persuading you to swing to one side or another, I'm saying as an alternate perspective to why TPG over MyRepublic, as compared to @Jurong640's MyRepublic over TPG.