I think is not necessary, I had test run before signed bundle modem at my phone and B618 and result is acceptable. The key is the bundle modem is.... not so....well.
After sign up with bundle modem. Each time I had captured bundle modem, phone and B618 speedtest results to UM customer service and they will said no problem on tower/my area. Then UM CS will dispatch new router to me. (Of cause MUST logged with MCMC).
The strong point is my entry level 5G phone able get average 200mbps all time at most of spot of my home. For the bundle modem I did used 10feet ladder trying every indoor location, moving modem
inch to inch,
step by step,
up and down and
turn a round to get best B28 and N78 signal strength and SINR. Once moment it able get >100mbps, next moment the signal strength unchanged but performance will drop below 10mbps.
Even at B618 locked B28 only the performance better than UM's modem.
Meantime, I also tested 5G WIFI only and linked external WIFI AP 5G only(Include different LAN cable). The result remain same.
Sometime I always question "mana wire pasang salah". Exchange the upload speed with download speed, at least I can run some thing.

I have similar issue like this. Once I get a good speed, not changing the modem's location, but suddenly speed drop. When use phone the speed is really fast. I think because of the cheap modem provided by UM. Guess i will buy a new modem that can pull signal better.