My own router?

I cancelled xfinity after my 2 years expired on April 18th.  I switched to Verizon home internet afterwards.  I put the square router box from them in ip passthrough mode.  The router light yutns green.  Then I plug my existing links velop into eternity port number 2 and I am good. I have 1 MX5300 and 3 MX4200 velops.

I have tmobile magenta (not max) 55+.  I went to a street carnival I guess you could call it.  I saw a tmobile booth there.  He sais if I switch to max plan it can be only 25 bucks a month.  Or only 40 if I have the regular plan not 50 since I already use them for cell service.  Sounds good.

He also said my existing velop just plugs right in to thr back of the tmobile black router box.  I though tmobile did not allow that.  I saw youtube videos also saying no bridge mode or whatever exists.  Was the guy I talked to wrong?  Or tmobile finally allows my own router or mesh setup?