You can buy iron on seam tape (similar to that used in goretex jackets)from profabrics.co.uk
Easy enough to apply with a house hold iron, low to medium heat to start with and a piece of grease proof paper over the top to stop any glue wrecking the iron.
Former "Seamgrip" was PU based. It's now called "Seamgrip + WP".
Former "Silnet" was silicone based. It's now called "Seamgrip + Sil".
PU based glues don't work on silicone coated fabric, no matter if it's silpoly or silnylon (unless the fabric is worn so well, there's hardly any silicone coating left on/in it, then it's different obv.). Initially it may look okay, but once you manipulate a silicone treated fabric with a blob of PU glue, the blob will eventually come undone.
Should your fabrics be silicone coated, you'll have to use silicone based glue.
My Wildcat drybags always were PU coated (on the inside). One is from about 2017 and the other from a couple of years later - they were slightly different in some aspects, but both PU coated, both with many fixed holes and replaced tape - all done with the stuff suited for PU applications.
If I have to waterproof a seam, I dilute the glue suited for the fabric I want to work on about 1:5 in cleaning petrol/benzine. If I need to reproof a large area eg of a tent floor or tent fly, I dilute the stuff roughly to 1:20 and work outside.
If you have large surfaces, you can also use the silicone you may have lying around from the DIY shop.
New bag turned up from Wildcat this morning. I actually prefer it to the old one because it is slightly larger, and the closure stiffeners are much better. For some daft reason the old bag's ones bent away from each other which made closing it a right pain, but those on the new one lie flat against each other. Yes I know it was designed so that you have the choice of attaching the bag attachment straps directly to the harness side clips OR to close the bag with them and use the separate harness clips separately around the bag. But that doesn't explain the daft bent stiffeners.
"My God, Ponsonby, I'm two-thirds of the way to the grave and what have I done?" - RIP
"At least you got some stories" - James Acaster
"A little nonsense now and then is relished by the wisest men" - WW