So here is the stats I got Rain LTE VS Cellc LTE+/4G+
Rain LTE/4G:
Band 3 FDD: 1800MHz has 10MHz bandwidth
Band 38 TDD: 2600MHz has 20MHz bandwidth
Rain does NOT have carrier aggregation between these 2 bands which is sad really because combined bandwidth of 30MHz is a decent size that would provide some very decent speeds when aggregated.
Cellc LTE and LTE+/4G and 4G+:
Band 3 FDD: 1800MHz has 10MHz bandwidth Secondary servicing cell usually.
Band 1 FDD: 2100MHz has 5MHz bandwidth

Primary servicing cell usually
So when forcing the B618 to each specific band on Cellc you can see it stays on 4G only but Once you combined them either by allowing all LTE bands or by forcing only band 1 and 3 you can see the 4G+ sign coming up and cellc activates 2ca on the B618. Upload can be set to either 2100Mhz or 1800Mhz respectively.
So Cellc has 2CA on their 1800Mhz and 2100Mhz LTE bands giving a combined bandwidth of 15Mhz explains the slow speeds on Cellc LTE+/4G+ compared to Rain 2600Mhz LTE only.
Damn I love playing with the networks bands and seeing how the networks work with LTE

On a different note. Shame on you Rain for misleading the public. Get CA right and then we will talk.